Water-based mobs typically spawn in oceans or in the case of the turtles on beaches. All these mobs only appear in the overworld, due to the overworld being the only dimension with oceans. Aquatic mobs have swim mechanics and can move freely in water on the x, y, and z axis. Drowned are not included due to being a modified zombie and can't swim.
How many aquatic mobs are there in Minecraft
11 mobs according to our database selection.