Plesiosaurus

Hardy. Any critical hit against the dinosaur becomes a normal hit.

Hold Breath. The plesiosaurus can hold its breath for 1 hour.

Multiattack. The plesiosaurus makes two Bite attacks.

Bite. _Melee Weapon Attack:_ +6 to hit, reach 10 ft., one target. _Hit:_ 11 (2d6+4) piercing damage.

Flipper Slam. While underwater, the plesiosaurus slams its flippers together, sending a shockwave of water in a 15-foot cone. Each creature in the area must succeed on a DC 13 CON save or be off-balance until the end of its next turn. While off-balance, a creature's speed is halved, and it has disadvantage on the first attack roll it makes on each of its turns.