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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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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.

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