An Underwater Hostile Mob

Doesn't have to be a shark it could be a fantasy mob, one that is fast moving, melee attacking, yet really slow times between attacks that can't attack you if you are on land or on a boat. That spawn in the oceans or in large aquified caves, or it could have a generated structure or landmark to indicate where it will spawn

Why: for a challenge or variety for the underwater as only drown and drown trident are really a threat underwater, guardians and elder guardian only spawns in ocean monument which is quite rare