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What happens when you cast the fireball spell underwater? And does the water remain?
Assume a perfectly spherical pool with a 20 foot radius and hole at the top for you to cast the fireball into the pool with. There is a goblin in the center of the pool with 1 HP. This means the fireball will be igniting in the exact center of the pool, underwater.
Completely transparent glass in a brightly lit room.
Unless there are some secret rules written away in the DMG or PHB that details how water affects magical and elemental occurrences, the fireball should occur normally.
I think earlier editions had rules where lightning would spread out in an AOE (area of effect), but as far as I know that doesn't exist in 5e. This isn't to say fire would spread in a larger AOE under water, but to show that there have been situations where magic is affected by water.
But seeing as the spell doesn't detail anything happening if the spell is submerged or the area the spell occurs is covered in water, the spell should progress as normally.
A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame. Each creature in a 20-foot-radius sphere centered on that point must make a dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.
The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.
The good news for the goblin is he will be taking half damage. PHB 198 says...
Creatures and objects that are fully immersed in water have resistance to fire damage.
Outside of that fireball seems to still work in full.
What happens to the water though? It just had fire flood all around the inside of it.
Rules as written: It doesn't say or really hint or imply that anything would happen. With this in mind I'd say the water is still there. Perhaps it bubbled violently and got a bit steamy. Though with how brief the fire lingers in an area I imagine the water would stay. This is more of an opinion but I base it around that the PHB provides no insight on what would happen.
You could consider the water 'ignitable' and that it steams away but that's a choice as DM.
Official ruling to Does Fireball evaporate water?:
Nothing in the rules causes a fireball to vaporize water. Magic ≠ physics. DMs may apply whatever magical/scientific logic they like. — Jeremy E. Crawford
With regard to fireball: Creatures and objects that are fully immersed in water have resistance to fire damage. (PHB 198) It does not help the hopeless goblin though ...
You could also consider that water could create an obstacle (PHB 204 - for spells that require a clear path to the target). Following that idea - you could theoretically rule that fireball cast outside water explodes when it hits the water surface. (Rather harsh ruling IMHO)
You need to remember one thing. Spells aren't physics, they just replicate some effects without following the rules of nature. Indeed thats why they are magical spells.
As spellcasting states
A spell is a discrete magical effect, a single shaping of the magical energies that suffuse the multiverse into a specific, limited expression.
Specific, limited. expression is the key here. A fireball works underwater cause there is no clause in the spell says it doesn't and that's the limited specific effect the spell does. Same reason it doesn't put objects you are holding on fire, even if you are holding a rag soaked in tar, or why it only ignites objects instead of dealing damage to them
