Rules as written, I can't use cold of cone to freeze water. But can I cast a wall of water underwater, then use a cold damage spell on it, would it freeze just that section of water?

Also usually a creature can hold its breath underwater to keep from drowning. If I cast Hideous Laughter, and they start to laugh and lose their air as they fail, do they just start to drown?

Pretty much all of this can be answered with three simple words: "Ask your DM". Underwater combat is not a particularly developed system in D&D because people rarely engage in it, partly because it produces all kinds of weird scenarios like these that slow down a session as people try to work out the answers.

If you're abiding solely by RaW, then cone of cold cast on wall of water wouldn't do anything to nearby areas of water. Although arguably, you wouldn't even be able to cast wall of water underwater; it's debatable whether or not the ocean floor is "the ground".

There's nothing in RaW about hideous laughter affecting breathing, so the answer is entirely DM-dependent. I'd rule that the affected creature loses a minute of breath each round.

I would rule that you can, in fact, cast cone of cold underwater, provided you can speak the necessary verbal components and you can move for the needed somatic ones, and the small glass cone isn't whisked away by a current.

I mean, cone of cold creates cold air that blasts out from your hands. There's no real reason it couldn't be cast underwater. The air would push the water out of the way, freezing around the edges, eventually creating a cone of air surrounded by a skin of ice.  Might be handy for drowning victims immune to cold damage. Could clear an underwater combat arena, kill coral, and more.

That's my thinking, though,; the big issue is the verbal component -- which is not a problem that is about this spell specifically, but about any spell requiring verbal components. Hard to speak the words properly underwater and not need to breathe before or after.

Quote from AEDorsay >>

I would rule that you can, in fact, cast cone of cold underwater, provided you can speak the necessary verbal components and you can move for the needed somatic ones, and the small glass cone isn't whisked away by a current.

I mean, cone of cold creates cold air that blasts out from your hands. There's no real reason it couldn't be cast underwater. The air would push the water out of the way, freezing around the edges, eventually creating a cone of air surrounded by a skin of ice.  Might be handy for drowning victims immune to cold damage. Could clear an underwater combat arena, kill coral, and more.

That's my thinking, though,; the big issue is the verbal component -- which is not a problem that is about this spell specifically, but about any spell requiring verbal components. Hard to speak the words properly underwater and not need to breathe before or after.

 

It's a fine ruling, kind of based on "reality" vs D&D, but if you are going to go that way, the ice formed is unlikely to be thick enough to withstand the pressure of the water so if it formed an air cavity it would likely immediately collapse.

Personally, I allow pretty much all spells to be cast underwater assuming the spell caster has the ability to breath underwater.If the spell caster has no way to breath underwater then I would have a skill check involved in casting spells with verbal components (house rule since I don't think the PHB/DMG mention it). I'd then adjudicate any interactions that might come up.

Quote from AEDorsay >>

I would rule that you can, in fact, cast cone of cold underwater, provided you can speak the necessary verbal components and you can move for the needed somatic ones, and the small glass cone isn't whisked away by a current.

I mean, cone of cold creates cold air that blasts out from your hands. There's no real reason it couldn't be cast underwater. The air would push the water out of the way, freezing around the edges, eventually creating a cone of air surrounded by a skin of ice.  Might be handy for drowning victims immune to cold damage. Could clear an underwater combat arena, kill coral, and more.

That's my thinking, though,; the big issue is the verbal component -- which is not a problem that is about this spell specifically, but about any spell requiring verbal components. Hard to speak the words properly underwater and not need to breathe before or after.

 

My character is a Triton enchantment wizard so talking/breathing isn’t an issue. This is just the first time underwater combat has came up and it makes spell casting unusual.

A spell that can turn a living person into a frozen statue should certainly also be powerful enough to freeze water. After all, living creatures almost invariably have both a higher body temperature than water, and some sort of outer coating - clothing, or fur, or whatever - that protects against rapid loss of temperature. Water does not.

You have two choices underwater.

1. Suspend belief/reflavor things to just work as the rules say they work.
2. Write up alternate rules for about half the spells in the game.

The devs picked #1 because #2 gets real crazy real quick, and as is already apparent 5 different people will have 5 different ideas about how Cone of Cold should interact underwater.

It's more fun to just play the game rather than stop at every spellcaster's turn so the DM can figure out new rules. Especially if you don't happen to be a spellcaster. Trying to apply physics to magic is fun when you're kicking around the forums, but in an actual game it just grinds things to a halt and/or introduces precedents that eventually - if not immediately -  break the game.