(partly cribbed from 5eSRD)
When making a melee weapon attack, a creature that doesn’t have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.
A ranged weapon attack automatically misses a target beyond the weapon’s normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).
Creatures and objects that are fully immersed in water have resistance to fire damage.
Nonmagical fires (ie ongoing damage from being set ablaze) are cancelled out by water.
Initial acid damage is unaffected, but ongoing acid damage is halved or negated by water.
Lightning tends to spread out as it seeks a ground. Anyone within five feet of a target that takes lightning damage must make a DC 10 Dex save or take 1d6 lightning damage.
Thunder is much louder and travels much farther underwater. Any creature that takes thunder damage underwater must make a DC 10 Con save or be deafened until the start of their next turn. Thunder based spells that make a loud noise have their audible range quadrupled.