Hold Breath. The plesiosaurus can hold its breath for 1 hour.
Actions
Bite.Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 14 (3d6 + 4) piercing damage.
Running this fight? Add the plesiosaurus to the initiative tracker: type its name and the AC, hit points and initiative bonus fill in, and the book button shows this stat block mid-fight.