There are a number of spells that produce their effects by conjuring and then manipulating large volumes of water, including Watery Sphere, Tidal Wave, and Tsunami. As far as I can tell, nothing stops someone from casting these spells underwater (assuming they are able to cast spells underwater, of course). However, it is not clear what effect would be produced by conjuring a mass of water in an area where there is already water. Would the spells have no effect? Would they have their normal effects? If so, what do those effects look like? What will happen when one of these spells is cast underwater?
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asked May 26, 2019 at 17:55
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A spell only does what it says in the description.
None of the spells you list would be affected by being underwater. All of the spells' effects occur. However, Watery Sphere can hover but no more than 10 feet off the ground. There may be some contention about what "off the ground" entails.
Underwater combat rules can be found in the PHB (198) which includes modifiers to weapon attacks but nothing specific regarding spells (creatures and objects immersed in water have resistance to fire damage, difficulties for V and S component use may arise).
For spellcasting you should also consider Visibility Underwater as explained in the DMG (117).
Visuals of underwater-effects at my table.
Following that there is displacement conjured by some form of force, I as the DM narrate this as the conjured water displacing the already present water - pushing it out to all sides or in the direction of the force. In the case of Tsunami that is away from you. In the case of Tidal Wave that is downwards. In the case of Watery Sphere that is to all sides equally.
We also once tried to find out, same table, what the path of least resistance would be, this included lots of temperature calculations considerations of pressure, etc. and left us with an overwhelming amount of unknown variables - we decided that my initial way to narrate the effects was more fun for our group, but maybe you/your group is interested in this approach.
answered May 26, 2019 at 18:13
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\$\begingroup\$ Since water is functionally incompressible, the path of least resistance is usually towards the nearest quantity of compressible air. That's almost always directly up, which is why underwater explosions create the huge vertical column of water that we all think of as the "depth charge" visual. That's also why depth charges work -- if you set off an explosion like that underwater near the compressible air inside a submarine, the water will try to compress the submarine before it tries to lift the entire water column out of the way. \$\endgroup\$ Commented Aug 5, 2021 at 20:31
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\$\begingroup\$ @DarthPseudonym yes, and no. It gets super complicated, especially when you don't have the interface of water and structure and look at things like violent fluid-conjured force interaction and fsi problems are also much more complex than "it goes up." \$\endgroup\$– AkixkisuCommented Aug 5, 2021 at 22:09