Adult Hydro-Serpent

Hold Breath. The Adult Hydro-Serpent can hold its breath for 5 hours.

Underwater Camouflage. The Adult Hydro-Serpent has advantage on Dexterity (Stealth) checks made while underwater.

Brute. A natural weapon attack deals one extra die of its damage when the Adult Hydro-Serpent hits with it.

Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 29 (3d12 + 8) Piercing damage. The target is then Grappled with an escape DC of 18. Until the grapple ends, the creature is restrained and the young sea serpent can't bite another target.

Constrict. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 23 (3d8 + 8) bludgeoning damage, and the target is Grappled with an escape DC 18. Until this grapple ends, the creature is restrained, and the serpent can still constrict another target.

Frigid Breath (Recharge 5–6). The Sea Serpent exhales freezing gas in a 30-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 50 (10d8) Cold damage on a failed save, or half as much damage on a successful one.