I know that RAW being underwater doesn't effect spells unless the spell says so but I'm interested in if anyone has any fun house rules for dealing with different damage types underwater.

I say that creatures have resistance to fire and thunder damage while underwater, and have disadvantage on saving throws against taking lightning or cold damage while underwater.

The rules already state that creatures have resistance to fire damage underwater unless the effect producing the damage ignores this (like the dragon turtle's breath weapon).

Given that sound propagates far more easily underwater, the idea that you should get resistance to thunder damage while submerged makes no sense.

thank you for the information i will correct this

I don't like to just give a flat no to things, but I don't see how lightning should be controllable at all underwater. Perhaps the spell includes a channel that contains the lightning or a point that overwhelmingly draw the current? I mean it shouldn't work that great in a room full of metal weapons and armor either.

Acid shouldn't really be that effective as it is quickly diluted. Some kinds of poison attacks as well.

As for a Fireball, I would be tempted to do half damage BUT mark the area as superheated water, and if a creature ended its turn still in the area, it would take the other half.

What about Stinking Cloud? Does water breathing include water smelling? What if you're wearing your own source of air like a Breathing Bubble?

This stuff is fun to think about, but in reality I'm just going to adjudicate on the spot because you'd need a whole lot of extra rules and cases to write it all up.

Stinking Cloud and other cloud effects probably shouldn't function underwater.

I'd only consider houserules where the elemental damage is occurring from a physical mundane source. With magic, I wouldn't bother - the magic compensates in most cases, because it's magic.

Inventing a bunch of rules seems like a pointless endeavour unless you're planning a mostly underwater campaign. A few ad hoc adjudications, mostly within the sphere of "advantage/disadvantage" here and there (as seems the design intent) is plenty, because going too far into it would just be a rabbit hole of houserules that would just overcomplicate things. 

Quote from 6thLyranGuard >>

Stinking Cloud and other cloud effects probably shouldn't function underwater.

 Water creatures use smell, why would they not be affected by stinking cloud?

Cloudkill doesn't care if you have to breath or not.

Quote from Wysperra >>

Quote from 6thLyranGuard >>

Stinking Cloud and other cloud effects probably shouldn't function underwater.

 Water creatures use smell, why would they not be affected by stinking cloud?

Cloudkill doesn't care if you have to breath or not.

 
Just to add and expand on this: Waterbreathing let's you "breathe" the water - so you still inhale through mouth and nose, taking water into yourself, which would include anything in that water, like the poison from Stinking Cloud / Cloudkill. Theoretically humans can smell underwater - our olfactory glands could still detect odours. We just tend not to put this in practice because in the real world this results in our immediate drowning. But the odour particles are still present and still detectable by our glands so if you could inhale water via nostrils without drowning - yes, you could smell underwater. As Wysperra says, some creatures do smell things underwater. Sharks in particular are known for their keen sense of smell in water, able to detect even small amounts of blood from very far away.

Not all substances dissolve easily in water. Sometimes making a cloud in water will result in that cloud lasting longer than it would in air. So, even from a real world basis there's no reason to think a poison gas cloud released underwater would function less - since in some cases it can actually prove more lethal.

I would expect that the vapors in most cloud spells to rise to the surface and create a slick since they're in an aeresolized state when the spell is cast.

One could consider giving disadvantage against thunder and/or force damage considering how much more devastating such effects are in environment where the surrounding does not compress as well as air.

Quote from Kotath >>

Quote from Lathlaer >>

One could consider giving disadvantage against thunder and/or force damage considering how much more devastating such effects are in environment where the surrounding does not compress as well as air.

 

As others have said the physics are completely the opposite since water carries shockwaves far more efficiently than air.

Em - yeah? It sounds like you are disagreeing but what I said is essentially the same. Air compresses better than water hence shockwaves deal much more damage underwater. That's why I said disadvantage against thunder/force damage. Unless you think I meant disadvantage for the caster? I meant disadvantage to Saving Throw against those effects :)

Quote from Kotath >>

Quote from scatterbraind >>

I don't like to just give a flat no to things, but I don't see how lightning should be controllable at all underwater. Perhaps the spell includes a channel that contains the lightning or a point that overwhelmingly draw the current? I mean it shouldn't work that great in a room full of metal weapons and armor either.

Acid shouldn't really be that effective as it is quickly diluted. Some kinds of poison attacks as well.

 

 

Again, if going with science, then lightning's effect depends on salt water vs fresh. Fresh water actually insulates. 

 Pure H2O insulates.  A freshwater stream or lake will have enough dissolved ions in it that it will be perfectly conductive unless there's something magical about it.