a fog cloud and an archer walk into a bar...

I say Full cover is like a hollow tree. Anything that is at least a foot thick is all dimensions and is solid/sturdy and fully surrounds you is full cover.

Quote from SmiteMakesRight_3_5 >>

As such, your statement, "if something is too opaque to see into you shouldn't be able to see through to the other side as your line of Sight is blocked", is incorrect except for certain, specialized cases.

 What would be such specialized cases if not Fog Cloud, dense fog, heavy snow, Darkness etc ? On the contrary i believe its correct for most cases except nonmagical Darkness. The thing is, all Heavily Obscured treated equal create problem depending of interpretation.

• So i prefer to treat all areas of Heavily Obscured as opaque blocking Line of Sight except for nonmagical Darkness that can see light.
• Then to treat all areas of Heavily Obscured as opaque not blocking Line of Sight so nonmagical Darkness can see light. 

I've been ruling this way since 2014 and work fine for my campaigns. 

Quote from SmiteMakesRight_3_5 >>

Quote from Plaguescarred >>

Quote from Unclevertitle >>

Quote from Plaguescarred >>

To me if something is too opaque to see into you shouldn't be able to see through to the other side as your line of Sight is blocked. 

 
That's directly intuitive when talking about opacity. (Fog Cloud)

But counter-intuitive when talking about luminosity. (Darkness)

The rules, however conflate both into Heavily Obscured, and here we are.

 I don't find it counter-intuitive for Darkness spell since nonmagical light can't illuminate it.

But it is for nonmagical Darkness being opaque like any other source of Heavily Obscured areas. So i prefer to treat category of illumination from such absence of light differently than other source or phenomenon, being an exception which can be illuminated by light thus possibly seen into.

Darkness isn't the only way to create an area of Darkness. A common way is just being dark. If you are in a large, dark room or space (such as a dungeon or cave), the area between two light sources (such as torches) would constitute Darkness but would not block sight between each other as long as those spaces are empty.

As such, your statement, "if something is too opaque to see into you shouldn't be able to see through to the other side as your line of Sight is blocked", is incorrect except for certain, specialized cases.

the area between two light sources (such as torches) would constitute Darkness is not true if I understand you.   If you are in a normal non magical room, you are not in darkness as it is defined.  The room is dark is not the same as being in the defined darkness.  Dark is the absence of light.  Period.  Darkness is a heavily obscured area.  That is a huge difference.

Those two light sources can be seen and therefore you are not in an heavily obscured situation.  If you were in a heavily obscured area, then you could not see those two light sources.

All Darkness is Heavily Obscured wether magical or not.

Darkness. Darkness creates a Heavily Obscured area. Characters face Darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon, or in an area of magical Darkness.

For example, if you and your ally are in Bright Light and an enemy in an area in between is;

A dense fog, heavy snow or Fog Cloud, then from where you are, it's illogical to me that you can't see the enemy yet see your ally further behind it. The opaque area come first in your Line of Sight,  which is blocked being unable to see through.

Same for Hunger of Hadar or Darkness spell, unless you can see through it somehow with feature such as Devil's Sight, you won't be able to see your ally on the other side.

But if the area is nonmagical Darkness instead, then it make sense to me that you can't see the enemy yet see your ally further away in Bright Light. You'd also see the enemy with Darkvision. It's the ruling i think make most sense of it all but that's just me.

Quote from Plaguescarred >>

For example, if you and your ally are in Bright Light and an enemy in an area in between is;

A dense fog, heavy snow or Fog Cloud, then from where you are, it's illogical to me that you can't see the enemy yet see your ally further behind it. The opaque area come first in your Line of Sight,  which is blocked being unable to see through.

Same for Hunger of Hadar or Darkness spell, unless you can see through it somehow with feature such as Devil's Sight, you won't be able to see your ally on the other side.

But if the area is nonmagical Darkness instead, then it make sense to me that you can't see the enemy yet see your ally further away in Bright Light. You'd also see the enemy with Darkvision. It's the ruling i think make most sense of it all but that's just me. 

 You're not the only one. That's my ruling too.

Quote from SunIsGettingRealLow96589 >>

So what the heck is blindsight? 

My understanding is something like Tremorsense isnt blocked by normal line of sight obstacles, but can see anything standing on stone.

Blindsight kinda says its not based on light, unaffected by darkness, but can it see THROUGH things that light can see through?  Like a bats echolocation probably doesnt travel through glass. But can blindsight see though glass? Can blindsight see through things opaque to light?  Like tremorsense doesnt care about the cave walls, it can "see" whats on the other side. It can "see" through objects standing on stone.

Not sure how blindsight fits into this 

"Blindsight

If you have Blindsight, you can see within a specific range without relying on physical sight. Within that range, you can see anything that isn’t behind Total Cover even if you have the Blinded condition or are in Darkness. Moreover, in that range, you can see something that has the Invisible condition."

Blindsight cannot see through anything that is behind total cover. 

"A target with total cover can’t be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle."

Though I don't think "obstacle" is defined in the rules, here we can assume that an "obstacle" is normally some kind of object, creature, physical environmental feature, etc. The devs have suggested in the past, I believe, that glass, though transparent, does provide total cover. The opacity of an object does not change the amount of cover it provides. So in this case, Blindsight cannot see through "objects opaque to light", because Blindsight cannot see through objects. 

However, I would rule that it would be able to see something through, say, a Wall of Fire. The spell says the wall of fire is "opaque", but not that it provides cover. In this case, because the fire isn't a physical barrier, I would say Blindsight would be able to see through the Wall of Fire to what is on the other side.

I'd say deep sea is likely nonmagical Darkness being pitch black past a certain distance underwater.

I think deep sea nothing but blindsight and Truesight can see through. Creatures that live down there have to have blindsight to survive, or else produce their own light (like anglerfish).

IMO most sea creatures such as Octopus or Piranha should be able to see with Darkvision in the deep sea.

Yeah, not all, but a lot of beasts with Swim speed have Darkvision (just for adding one more: a Frog). Similar with other creature types with Swim speed.

A Giant Crab or Giant Shark, for example, doesn't have Darkvision, but they have Blindsight instead.

Yes. 

Quote from Plaguescarred >>

IMO most sea creatures such as Octopus or Piranha should be able to see with Darkvision in the deep sea.

 Makes sense. Maybe they have a special kind of dark vision that could be implemented?

Quote from LinkArcher >>

Yes. 

Quote from Plaguescarred >>

IMO most sea creatures such as Octopus or Piranha should be able to see with Darkvision in the deep sea.

 Makes sense. Maybe they have a special kind of dark vision that could be implemented?

 There's only one Darkvision special sense.

Quote from SunIsGettingRealLow96589 >>

wizard casts Fog Cloud on the goblin. The area around the goblin is heavily obscured. The goblin has the blinded condition.

next in initiative, the ranger wants to fire their longbow at the Goblin.

heavily obscured says: You have the Blinded condition while trying to see something in a Heavily Obscured space. the Blinded condition says: "Attacks Affected. Attack rolls against you have Advantage, and your attack rolls have Disadvantage."

So, the goblin is blinded, therefore attack rolls against it have Advantage. The Ranger is blinded so attack rolls are at disadvantage.

advantage and disadvantage cancel out.

result: the Ranger shoots their arrow at the goblin,  who they cannot see, with a normal 1d20 attack roll.

Did I miss anything?

 I did not read the entire thread before replying to the OP.

On the surface it would appear that the advantage and disadvantage cancel out if both player and goblin are in the fog together.

From a DM perspective I would say the advantage and disadvantage might not cancel each other out because heavily obscured is not the same as blinded.  But I might be wrong.  I do not have the rules right in front of me.

In any case, “Blind Sight” from either a fighting style, a class/subclass feature, or from a Familiar’s senses, would give the ranger the upper hand on the goblin.

Quote from packman767 >>

[...] From a DM perspective I would say the advantage and disadvantage might not cancel each other out because heavily obscured is not the same as blinded.  But I might be wrong.  I do not have the rules right in front of me. [...]

 Not the same, but related:

You have the Blinded condition while trying to see something in a Heavily Obscured space. See also “Blinded,” “Darkness,” and “Playing the Game” (“Exploration”).

Blinded:

[...] Attacks Affected. Attack rolls against you have Advantage, and your attack rolls have Disadvantage.

Quote from Plaguescarred >>

Quote from LinkArcher >>

Yes. 

Quote from Plaguescarred >>

IMO most sea creatures such as Octopus or Piranha should be able to see with Darkvision in the deep sea.

 Makes sense. Maybe they have a special kind of dark vision that could be implemented?

 There's only one Darkvision special sense.

 There should be a different one for deep sea.

Quote from LinkArcher >>

Quote from Plaguescarred >>

Quote from LinkArcher >>

Yes. 

Quote from Plaguescarred >>

IMO most sea creatures such as Octopus or Piranha should be able to see with Darkvision in the deep sea.

 Makes sense. Maybe they have a special kind of dark vision that could be implemented?

 There's only one Darkvision special sense.

 There should be a different one for deep sea.

 Why do you think there should be a different Darkvision for deep sea?

The Darkness at deep sea, called ''Midnight zone'' is due to the absence of light, similar to underground cavern.

Deep-Sea Visibility is categorized by 3 Zones:

• Euphotic Zone (0–200m): Sunlight zone where most photosynthesis occurs and visibility is best, though limited by algae and suspended sediments.
• Dysphotic Zone (200–1,000m): "Twilight" zone with minimal light penetration, where photosynthesis is not possible.
• Aphotic Zone (Below 1,000m): "Midnight" zone, permanent total darkness.