Combat
From the PHB:
Melee:
If you have am innate swimming speed (including temporarily through magic), things proceed as normal.
If not you have disadvantage unless using a dagger, javelin, shortsword, spear, or trident.
Ranged:
Regardless of swimming ability, all ranged weapons are limited to normal range. Most are at Disadvantage, except: a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).