Greater Dark Beast of Waves (5e Creature)
From D&D Wiki
Damage Vulnerabilities Lightning, Radiance
Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 63 (5d12 + 30) damtype damage. The beast attempts to bite of a chunk of the boat.
Tsunami. (Recharge 4-6). The powers of the beast causes a large 100 foot wave to hit the boat. The boat must make a DC 28 Constitution saving throw. It takes 112 (3d20 + 80) force damage on a failed save, or one-fourth as much damage on a successful one.
A giant leviathan like creature, almost 2 miles long, and 25 feet wide. It has a dozen webbed appendages it uses to propel itself through the water at rapid speeds each with a hooked claw at the end of each of them. With one giant mouth taking up all of the beasts head, that can open up to 60 feet to swallow something. The water around the beast will turn into a current propelling the creature faster in the direction that it is going. Its body made of water which seems takes the form of a scaly fish like creature, and though it is liquid it is almost feels hard like a solid.