Hold Breath. The plesiosaurus can hold its breath for 1 hour.
Maneuvers. The plesiosaurus uses maneuvers (maneuver save Difficulty 14). It has 7 stamina dice which are d8s, a dice limit of 1, regains all expended stamina dice when it finishes a long rest, and regains half its total stamina dice (rounded up) when it finishes a short rest. It knows the following maneuvers:
Bite. Melee Weapon Attack: +6 to hit, reach 2 m. Hit: 14 (3d6 + 4) piercing damage.
As an action, you can make a melee weapon attack against a creature after you move at least 4 meters straight toward it. Add the stamina die to the attack’s damage (add half if you miss by 4 or less). On a hit, if the creature is your size or smaller, the creature must succeed on a Reflex saving throw or you can push it up to 2 meters away from you and knock it prone.
You can increase the damage, target a creature of a larger size, and push the creature an additional 1 meter for each additional stamina die expended.
As an action, you can use the Dash action. If you move at least 4 meters straight toward a creature, you can make a melee weapon attack against it. Add the stamina die to the attack’s damage (add half if you miss by 4 or less).
You can increase the damage for each additional stamina die expended.