Aquatic

The term Aquatic refers to an in-game group of mobs that live in oceans and rivers and are able to swim.

Classification
They take extra damage from tridents enchanted with Impaling,‌ and, except axolotl, do not cause pufferfish to inflate nor take damage from them. All aquatic mobs except dolphins are immune to drowning, and all except guardians, elder guardians, and turtles take suffocation damage if out of water for too long. Aquatic mobs have the ability to swim, whereas other mobs simply float on water or sink. In Bedrock Edition there is no in-game classification for these mobs.

Trivia

 * Despite Drowneds being a underwater variant of Zombies, they are not considered aquatic mobs, but undead.
 * Corals and sea pickles instantly die if they get out of water for too long similar to the the aquatic mobs.