I think it has to do with fire being a chemical reaction rather than a substance. Since creatures like Salamanders and Fire Elementals can't fly by default, there needed to be ground. In previous editions of the game, you took fire damage simply from being on the plane unless you had magical protection. Five E does away with that and merely makes the plane a hot desert with vicious winds. Decidedly less impressive but I guess they had to tone it down since getting immunity to elemental damage is a lot harder in this edition than it was in 2nd or 3rd Edition so having everyone take 3d8 fire damage a round would make adventures on the plane virtually impossible. Of course, the Inner Planes have gone from having an entire sourcebook in Planescape to having a few paragraphs worth of blurb in the DMG.
Yeah, I guess I see "elemental elements (fire, air, earth, water)" being something a little more "essence" or "ideal" form laden than the substances on the Prime material plane. In other words, there is some sort of elemental fire that can be used as something solid to stand on, or even build structures out of for those immune to fire to thrive among. I'm not quite sure how to work this out mechanically, but it's where my multiverses "head canon" is going.
I'm now thinking of the Weather Dominator components from that 80s G.I. Joe cartoon ... since the planescape goes way off the deep end with the laws of physics anyway.
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Jander Sunstar is the thinking person's Drizzt, fight me.
The bigger issue is really being able to effectively use weapons in armor that covers the whole body. Most armor is not built to deal with water resistance whatsoever. There's a reason that most large undersea predators in the last 10 million years don't have limbs. It kills the creature's speed, and doesn't improve deadliness. Exceptions being creatures without articulations, like giant squid and octopi.
5E does have some penalties for weapons when underwater. Nothing super complex, but IMO like with armor, these are generally good enough bare bones penalties that don't bog things down by getting too specific as a baseline, and more realistic games can add more to it.
Underwater Combat
When adventurers pursue sahuagin back to their undersea homes, fight off sharks in an ancient shipwreck, or find themselves in a flooded dungeon room, they must fight in a challenging environment. Underwater the following rules apply.
When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.
I think the issue with this is that it makes races like Tritons (and Aquatic Elves whenever WotC decides to spend any time developing underwater adventures b/c you know there's nothing the devs love more than more Elf sub-races) OP in the water once they get their hands on heavy armor. All Fighter-type underwater races should be clamoring for chain mail/plate mail since, per RAW, they suffer no disadvantage from wearing it whatsoever under the current rules. If Triton and Merfolk are constantly fighting off Sahuagin and have better relations with above-water races that have smithing abilities (unlike the xenophobic Sahuagin), it would be ridiculous NOT to try to equip your shock troops with the best armor possible when it comes at no cost to mobility or lethality.
It makes iron equipment impractical for aquatic races like merfolk and tritons. You would not be able to build weapons out of iron or steel and have them retain a workable edge for any serious length of time. Such races would either use bronze tools since bronze doesn't rust, or they'd use non-metal equipment, including magical things like ironwood and glassteel, which no longer exist in the game but used to be important crafting material.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
It makes iron equipment impractical for aquatic races like merfolk and tritons. You would not be able to build weapons out of iron or steel and have them retain a workable edge for any serious length of time. Such races would either use bronze tools since bronze doesn't rust, or they'd use non-metal equipment, including magical things like ironwood and glassteel, which no longer exist in the game but used to be important crafting material.
Assuming chemistry works normally rather than it being more alchemical in nature. Pretty big assumption and begs the question why there are not advanced gunpowder weapons given there is no question of metallurgical technological advances.
Well, guns do exist in some D&D worlds and chemistry works normally for at least some things, like iron rusting.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
The bigger issue is really being able to effectively use weapons in armor that covers the whole body. Most armor is not built to deal with water resistance whatsoever. There's a reason that most large undersea predators in the last 10 million years don't have limbs. It kills the creature's speed, and doesn't improve deadliness. Exceptions being creatures without articulations, like giant squid and octopi.
5E does have some penalties for weapons when underwater. Nothing super complex, but IMO like with armor, these are generally good enough bare bones penalties that don't bog things down by getting too specific as a baseline, and more realistic games can add more to it.
Underwater Combat
When adventurers pursue sahuagin back to their undersea homes, fight off sharks in an ancient shipwreck, or find themselves in a flooded dungeon room, they must fight in a challenging environment. Underwater the following rules apply.
When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.
I think the issue with this is that it makes races like Tritons (and Aquatic Elves whenever WotC decides to spend any time developing underwater adventures b/c you know there's nothing the devs love more than more Elf sub-races) OP in the water once they get their hands on heavy armor. All Fighter-type underwater races should be clamoring for chain mail/plate mail since, per RAW, they suffer no disadvantage from wearing it whatsoever under the current rules. If Triton and Merfolk are constantly fighting off Sahuagin and have better relations with above-water races that have smithing abilities (unlike the xenophobic Sahuagin), it would be ridiculous NOT to try to equip your shock troops with the best armor possible when it comes at no cost to mobility or lethality.
Having an innate swimming speed helps a great deal. However, if you're going to wear heavy armor underwater you're still going to need:
A. Proficiency in heavy armor.
B. A strength score great enough not to gimp your movement from wearing heavy armor, depending on the specific armor. (Unless you spring for something rarer like mithral.)
So it's not like every sea elf or triton is immediately going to be trained and ready to fight at full capacity with heavy armor in the sea. Personally I think it's fine to have aquatic races have a distinct advantage in underwater combat, that's sort of their thing, but in my games underwater combat tends to be pretty situational anyway. I could see someone wanting to add more rules if they're playing a grittier or more reaslitsic game and plan to feature underwater areas frequently.
For a real world comparison lookup D Day and the first hand accounts of soldiers stepping off landing craft into deep water. Dozens of men just sank like stones and never came back up. These are strong fit men. Maybe 1% managed to get out onto the beach but they had nothing but their clothes and probably had a healthy dose of luck as well.
It makes iron equipment impractical for aquatic races like merfolk and tritons. You would not be able to build weapons out of iron or steel and have them retain a workable edge for any serious length of time. Such races would either use bronze tools since bronze doesn't rust, or they'd use non-metal equipment, including magical things like ironwood and glassteel, which no longer exist in the game but used to be important crafting material.
If there is a strong incentive to find and modify materials that are suitably water-proof, and enough intelligent minds that want to make it happen (not to mention presence of magic), why would we assume that somebody hasn't found a way to make rust-resistant or rust-proof armor? Incentive comes from the underwater humanoids being willing to trade all kinds of pearl, coral, and possibly unusual craft items to land-dwellers in exchange for stuff that said underwater societies cannot forge due to being surrounded by water most of the time. Not to mention, we already know that it's possible to make a full suit of armor from non-metal materials: dragon-scale. And if dragon-scale is too rare, why not use the chitinous exoskeletons of giant scorpions, purple worms, ankhegs, etc.? The point is, there is no reason that a fantasy universe where adventurers often fight creatures with an effective AC as high or higher than themselves should be without rust-proof material for making armor unless the DM is somehow dead set against the idea.
I think the issue with this is that it makes races like Tritons (and Aquatic Elves whenever WotC decides to spend any time developing underwater adventures b/c you know there's nothing the devs love more than more Elf sub-races) OP in the water once they get their hands on heavy armor. All Fighter-type underwater races should be clamoring for chain mail/plate mail since, per RAW, they suffer no disadvantage from wearing it whatsoever under the current rules. If Triton and Merfolk are constantly fighting off Sahuagin and have better relations with above-water races that have smithing abilities (unlike the xenophobic Sahuagin), it would be ridiculous NOT to try to equip your shock troops with the best armor possible when it comes at no cost to mobility or lethality.
Having an innate swimming speed helps a great deal. However, if you're going to wear heavy armor underwater you're still going to need:
A. Proficiency in heavy armor.
B. A strength score great enough not to gimp your movement from wearing heavy armor, depending on the specific armor. (Unless you spring for something rarer like mithral.)
So it's not like every sea elf or triton is immediately going to be trained and ready to fight at full capacity with heavy armor in the sea. Personally I think it's fine to have aquatic races have a distinct advantage in underwater combat, that's sort of their thing, but in my games underwater combat tends to be pretty situational anyway. I could see someone wanting to add more rules if they're playing a grittier or more reaslitsic game and plan to feature underwater areas frequently.
A. Per RAW, all Fighters (and a fair number of Clerics) are Proficient in heavy armor. Check.
B. Strength score that allows full movement in heavy armor. There's no reason why Triton or Merfolk Fighters should have less strength than their above-land counterparts. Especially because they swim most of the time, which arguably makes them stronger, not weaker. Therefore, a non-issue.
I never said that EVERY sea elf or Merfolk would have full plate. The point is that where IF there is any in-game consistency of intelligence and depth of thinking about implications of PHB rules and what would happen if those rules actually applied to humanoid beings with humanoid emotions at war with other humanoid beings, Tritons and Merfolks absolutely should WANT heavy armor for their more muscular melee warriors. Please read my text more carefully next time.
I've swum in a flak jacket and Kevlar and boots with a full pack and a rifle. There's a lot of ways for it to suck, but it is very doable. I would not say I'm a strong swimmer. This is literally a boot camp requirement - everyone has to do at least some degree of this - been a while, not sure the current minimum. Would metal armor be worse? Yes, I'm sure. But with proper technique, I'd be surprised (but not shocked) to find it impossible.
Always love seeing threads like this where there is no rule to punish a player for wearing heavy armor (which is already much worse than wearing lighter armors with high dex) and this.. need to impose some kinda of penalty on heavy armor even further.
Like ok.. so just dex build all your fighters than cuse these tables suck to play in heavy armor? Sleep in it, yer screwed, around water? your screwed. gotta buy it? your screwed. Caught unarmored? your screwed. Trying to sneak around? your screwed
As if dex wasn't already so much better, and light armors cheaper and not straight death sentences (heat metal? not in my leather armor)
Is it realistic? probably not but seriously lay off the poor SoB's wearing heavy armor they suck compared to dex guys already there's no need to slam them down even more
Always love seeing threads like this where there is no rule to punish a player for wearing heavy armor (which is already much worse than wearing lighter armors with high dex) and this.. need to impose some kinda of penalty on heavy armor even further.
Like ok.. so just dex build all your fighters than cuse these tables suck to play in heavy armor? Sleep in it, yer screwed, around water? your screwed. gotta buy it? your screwed. Caught unarmored? your screwed. Trying to sneak around? your screwed
As if dex wasn't already so much better, and light armors cheaper and not straight death sentences (heat metal? not in my leather armor)
Is it realistic? probably not but seriously lay off the poor SoB's wearing heavy armor they suck compared to dex guys already there's no need to slam them down even more
This is a good point too. The best heavy armor is only one AC higher than light armor with 20 dex, but far more expensive, takes longer to take off/put on etc. Not to mention what monks and bararians can do with unarmored defense with good stats. Generally the benefit of a high str class is using the bigger weapons without the finesse property for more damage rather than the armor itself, rather than the armor. I feel like heavy armor could use more benefits rather than more drawbacks.
It doesn't help either that, at least in my experience, ignoring encumbrance seems to be a somewhat common house rule. Though that's anecdotal on my end maybe it's not as common as the games I've played in make it seem.
For me, if someone is wearing plate armor, they are either:
A. A high strength character with proficiency, meaning they are both very strong and are trained in the use of said armor. So as a fantasy hero I'm willing to believe they swim in water with that armor, between their incredible strength and their training with said armor.
B. They lack one or both of those traits and as such the penalties associated with NOT having them will be compounded with the dangers of being underwater. But this seems incredibly rare, I'd have to go out of my way to contrive a reason for this to ever happen and I've never seen it happen in any of my games.
My paladin is STR 10, wearing mithril Heavy Plate (removes STR min); he worse ringmail before that, because Heavy Armor Master. So he will sink like a metal stone in water.
I would say if they are proficient and meet the strength requirement, yes I would say normal swim speed, but if they aren't proficient/don't meet the strength required to wear properly i would have them experience difficulties staying afloat, maybe make them do a strength (athletics) or constitution check
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Hollow unbreakable arrows are the most OP common magic item, and my current method of coming up with insane combat shenanigans.
if you make a steel pipe with one end closed and a nozzle on the other, you can enlarge it, fill with any liquid, and then drop concentration, creating a high pressure squirt gun. (or a pipe bomb, depending if it holds)
I would say if they are proficient and meet the strength requirement, yes I would say normal swim speed, but if they aren't proficient/don't meet the strength required to wear properly i would have them experience difficulties staying afloat, maybe make them do a strength (athletics) or constitution check
Yeah. Though in practice, it seems incredibly rare to me for someone to even put on heavy armor in the first place that they're not able to use normally, let alone try to swim in it anyway. I'm curious if anyone has actually had this happen, and if so what circumstances compelled the PC to put that armor on in the first place.
Wearing heavy armor when you are not proficient is a huge handicap. If all your weapon attacks are at disadvantage you are seriously nerfed and you can't cast spells. Might just abo
I would say if they are proficient and meet the strength requirement, yes I would say normal swim speed, but if they aren't proficient/don't meet the strength required to wear properly i would have them experience difficulties staying afloat, maybe make them do a strength (athletics) or constitution check
Yeah. Though in practice, it seems incredibly rare to me for someone to even put on heavy armor in the first place that they're not able to use normally, let alone try to swim in it anyway. I'm curious if anyone has actually had this happen, and if so what circumstances compelled the PC to put that armor on in the first place.
Wearing heavy armor when you are not proficient is a huge handicap. If all your weapon attacks are at disadvantage you are seriously nerfed and you can't cast spells. A one level dip into hexbalde would RAW prevent disadvantage but the disadvantage on dex saves and ability checks is also pretty bad. Might be just about playable for a Barb but still very sub optimal.
Not meeting the strength requirement is much less of an issue. A low strength wood elf (or other creature with a speed of 35ft) has the same walking speed as gnomes and dwarfs. I had a twilight cleric that I wanted to be pretty high int (and obviously Wis and con) who would only cast spells for attacks. Original plan was to wear heavy armor but I rolled well for stats so ended up with 14 Dex and medium armor.
It's not about weight. A pebble that wouldn't move the numbers on a scale would still sink to the bottom if thrown in the ocean. A cargo ship, made of metal, that weighs 220, 000 tons will float. It's about buoyancy. The Archimedes Principle. Someone else mentioned density or buoyancy in this thread way back, but no one addressed it and everyone involved kept arguing about weight.
In terms of realism, I'm pretty sure Lancelot and Beowulf swam fully armored in their stories. You might be able to safely ignore the density of heavy armor in your games. If your table doesn't care, then don't worry about it. As others have pointed out, heavy armor has enough built-in penalties. But for some reason, swimming in heavy armor breaks the suspension of disbelief for a lot of us, myself included. So I incorporate the rules that already exist into situations like swimming in heavy armor, but I try to keep it rules light and not too penalizing:
As a dm, I can require a skill check for situations that I think warrant it. In this case, I would ask for an Athletics check to swim in armor. If the armor imposes disadvantage to stealth checks, then this would apply to swimming also. Again, as a dm, I can impose disadvantage when I think it fits the situation. If they fail their check badly, I might have them sink 5' or even 10', or I might not, if I'm running a less gritty game. Characters could mitigate or remove these penalties by improving their buoyancy, like tying a half empty barrel to their back, fixing a bunch of empty waterskins to their waist, or adding strips of wood to their armor, whatever they can think of.
If someone is under the effects of the water breathing spell or the freedom of movement spell, this would also negate any swimming in armor penalties in my game. I rule that those spells and similar magic would make the characters buoyant enough to swim; since I'm adding a problem that doesn't exist in the D&D rules - buoyancy in this case - it's my responsibility to add solutions to that problem.
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Yeah, I guess I see "elemental elements (fire, air, earth, water)" being something a little more "essence" or "ideal" form laden than the substances on the Prime material plane. In other words, there is some sort of elemental fire that can be used as something solid to stand on, or even build structures out of for those immune to fire to thrive among. I'm not quite sure how to work this out mechanically, but it's where my multiverses "head canon" is going.
I'm now thinking of the Weather Dominator components from that 80s G.I. Joe cartoon ... since the planescape goes way off the deep end with the laws of physics anyway.
Jander Sunstar is the thinking person's Drizzt, fight me.
I think the issue with this is that it makes races like Tritons (and Aquatic Elves whenever WotC decides to spend any time developing underwater adventures b/c you know there's nothing the devs love more than more Elf sub-races) OP in the water once they get their hands on heavy armor. All Fighter-type underwater races should be clamoring for chain mail/plate mail since, per RAW, they suffer no disadvantage from wearing it whatsoever under the current rules. If Triton and Merfolk are constantly fighting off Sahuagin and have better relations with above-water races that have smithing abilities (unlike the xenophobic Sahuagin), it would be ridiculous NOT to try to equip your shock troops with the best armor possible when it comes at no cost to mobility or lethality.
I think the real issue there is that saltwater is a fantastically corrosive substance. Iron and steel don't last long in it.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
It makes iron equipment impractical for aquatic races like merfolk and tritons. You would not be able to build weapons out of iron or steel and have them retain a workable edge for any serious length of time. Such races would either use bronze tools since bronze doesn't rust, or they'd use non-metal equipment, including magical things like ironwood and glassteel, which no longer exist in the game but used to be important crafting material.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Well, guns do exist in some D&D worlds and chemistry works normally for at least some things, like iron rusting.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
This thread proves once more how there are two kinds of nerd in the world:
1) The kind who considers Force to be the energy field in Star Wars
2) The kind who believes Force = Mass x Acceleration
And that the two are seldom reconciled. :'(
Having an innate swimming speed helps a great deal. However, if you're going to wear heavy armor underwater you're still going to need:
A. Proficiency in heavy armor.
B. A strength score great enough not to gimp your movement from wearing heavy armor, depending on the specific armor. (Unless you spring for something rarer like mithral.)
So it's not like every sea elf or triton is immediately going to be trained and ready to fight at full capacity with heavy armor in the sea. Personally I think it's fine to have aquatic races have a distinct advantage in underwater combat, that's sort of their thing, but in my games underwater combat tends to be pretty situational anyway. I could see someone wanting to add more rules if they're playing a grittier or more reaslitsic game and plan to feature underwater areas frequently.
For a real world comparison lookup D Day and the first hand accounts of soldiers stepping off landing craft into deep water. Dozens of men just sank like stones and never came back up. These are strong fit men. Maybe 1% managed to get out onto the beach but they had nothing but their clothes and probably had a healthy dose of luck as well.
My DM warned that I (Pally) would drown if I missed a Jump check off a sinking boat because heavy armor sink me.
If there is a strong incentive to find and modify materials that are suitably water-proof, and enough intelligent minds that want to make it happen (not to mention presence of magic), why would we assume that somebody hasn't found a way to make rust-resistant or rust-proof armor? Incentive comes from the underwater humanoids being willing to trade all kinds of pearl, coral, and possibly unusual craft items to land-dwellers in exchange for stuff that said underwater societies cannot forge due to being surrounded by water most of the time. Not to mention, we already know that it's possible to make a full suit of armor from non-metal materials: dragon-scale. And if dragon-scale is too rare, why not use the chitinous exoskeletons of giant scorpions, purple worms, ankhegs, etc.? The point is, there is no reason that a fantasy universe where adventurers often fight creatures with an effective AC as high or higher than themselves should be without rust-proof material for making armor unless the DM is somehow dead set against the idea.
A. Per RAW, all Fighters (and a fair number of Clerics) are Proficient in heavy armor. Check.
B. Strength score that allows full movement in heavy armor. There's no reason why Triton or Merfolk Fighters should have less strength than their above-land counterparts. Especially because they swim most of the time, which arguably makes them stronger, not weaker. Therefore, a non-issue.
I never said that EVERY sea elf or Merfolk would have full plate. The point is that where IF there is any in-game consistency of intelligence and depth of thinking about implications of PHB rules and what would happen if those rules actually applied to humanoid beings with humanoid emotions at war with other humanoid beings, Tritons and Merfolks absolutely should WANT heavy armor for their more muscular melee warriors. Please read my text more carefully next time.
I've swum in a flak jacket and Kevlar and boots with a full pack and a rifle. There's a lot of ways for it to suck, but it is very doable. I would not say I'm a strong swimmer. This is literally a boot camp requirement - everyone has to do at least some degree of this - been a while, not sure the current minimum. Would metal armor be worse? Yes, I'm sure. But with proper technique, I'd be surprised (but not shocked) to find it impossible.
Always love seeing threads like this where there is no rule to punish a player for wearing heavy armor (which is already much worse than wearing lighter armors with high dex) and this.. need to impose some kinda of penalty on heavy armor even further.
Like ok.. so just dex build all your fighters than cuse these tables suck to play in heavy armor? Sleep in it, yer screwed, around water? your screwed. gotta buy it? your screwed. Caught unarmored? your screwed. Trying to sneak around? your screwed
As if dex wasn't already so much better, and light armors cheaper and not straight death sentences (heat metal? not in my leather armor)
Is it realistic? probably not but seriously lay off the poor SoB's wearing heavy armor they suck compared to dex guys already there's no need to slam them down even more
This is a good point too. The best heavy armor is only one AC higher than light armor with 20 dex, but far more expensive, takes longer to take off/put on etc. Not to mention what monks and bararians can do with unarmored defense with good stats. Generally the benefit of a high str class is using the bigger weapons without the finesse property for more damage rather than the armor itself, rather than the armor. I feel like heavy armor could use more benefits rather than more drawbacks.
It doesn't help either that, at least in my experience, ignoring encumbrance seems to be a somewhat common house rule. Though that's anecdotal on my end maybe it's not as common as the games I've played in make it seem.
For me, if someone is wearing plate armor, they are either:
A. A high strength character with proficiency, meaning they are both very strong and are trained in the use of said armor. So as a fantasy hero I'm willing to believe they swim in water with that armor, between their incredible strength and their training with said armor.
B. They lack one or both of those traits and as such the penalties associated with NOT having them will be compounded with the dangers of being underwater. But this seems incredibly rare, I'd have to go out of my way to contrive a reason for this to ever happen and I've never seen it happen in any of my games.
My paladin is STR 10, wearing mithril Heavy Plate (removes STR min); he worse ringmail before that, because Heavy Armor Master. So he will sink like a metal stone in water.
I would say if they are proficient and meet the strength requirement, yes I would say normal swim speed, but if they aren't proficient/don't meet the strength required to wear properly i would have them experience difficulties staying afloat, maybe make them do a strength (athletics) or constitution check
Hollow unbreakable arrows are the most OP common magic item, and my current method of coming up with insane combat shenanigans.
if you make a steel pipe with one end closed and a nozzle on the other, you can enlarge it, fill with any liquid, and then drop concentration, creating a high pressure squirt gun. (or a pipe bomb, depending if it holds)
Nah, they sink like a fish. But seriously, I'd do what's most fun or interesting for the story. It does depend on the group, of course.
Yeah. Though in practice, it seems incredibly rare to me for someone to even put on heavy armor in the first place that they're not able to use normally, let alone try to swim in it anyway. I'm curious if anyone has actually had this happen, and if so what circumstances compelled the PC to put that armor on in the first place.
Wearing heavy armor when you are not proficient is a huge handicap. If all your weapon attacks are at disadvantage you are seriously nerfed and you can't cast spells. Might just abo
Wearing heavy armor when you are not proficient is a huge handicap. If all your weapon attacks are at disadvantage you are seriously nerfed and you can't cast spells. A one level dip into hexbalde would RAW prevent disadvantage but the disadvantage on dex saves and ability checks is also pretty bad. Might be just about playable for a Barb but still very sub optimal.
Not meeting the strength requirement is much less of an issue. A low strength wood elf (or other creature with a speed of 35ft) has the same walking speed as gnomes and dwarfs. I had a twilight cleric that I wanted to be pretty high int (and obviously Wis and con) who would only cast spells for attacks. Original plan was to wear heavy armor but I rolled well for stats so ended up with 14 Dex and medium armor.
It's not about weight. A pebble that wouldn't move the numbers on a scale would still sink to the bottom if thrown in the ocean. A cargo ship, made of metal, that weighs 220, 000 tons will float. It's about buoyancy. The Archimedes Principle. Someone else mentioned density or buoyancy in this thread way back, but no one addressed it and everyone involved kept arguing about weight.
In terms of realism, I'm pretty sure Lancelot and Beowulf swam fully armored in their stories. You might be able to safely ignore the density of heavy armor in your games. If your table doesn't care, then don't worry about it. As others have pointed out, heavy armor has enough built-in penalties. But for some reason, swimming in heavy armor breaks the suspension of disbelief for a lot of us, myself included. So I incorporate the rules that already exist into situations like swimming in heavy armor, but I try to keep it rules light and not too penalizing:
As a dm, I can require a skill check for situations that I think warrant it. In this case, I would ask for an Athletics check to swim in armor. If the armor imposes disadvantage to stealth checks, then this would apply to swimming also. Again, as a dm, I can impose disadvantage when I think it fits the situation. If they fail their check badly, I might have them sink 5' or even 10', or I might not, if I'm running a less gritty game. Characters could mitigate or remove these penalties by improving their buoyancy, like tying a half empty barrel to their back, fixing a bunch of empty waterskins to their waist, or adding strips of wood to their armor, whatever they can think of.
If someone is under the effects of the water breathing spell or the freedom of movement spell, this would also negate any swimming in armor penalties in my game. I rule that those spells and similar magic would make the characters buoyant enough to swim; since I'm adding a problem that doesn't exist in the D&D rules - buoyancy in this case - it's my responsibility to add solutions to that problem.