Freeze. If the elemental takes cold damage, it partially freezes; its speed is reduced by 20 feet until the end of its next turn.

Multiattack. The elemental makes two slam attacks or one slam attack and one bite attack.

Slam. Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit: 13 (2d8 + 2) bludgeoning damage.

Bite: Melee Weapon Attack: +5 to hit, reach 15 ft, one target. Hit: 13 (1d12 + 2) Necrotic damage. If the Sea serpent bites a creature that is in water then that creature is swallowed and is in the belly of the serpent. The creature in the belly is blinded and has to do at least 10 damage cut an opening in order to get out of the body of the serpent. Whilst being attacked every turn by zombies for (2d4+2 bludgeoning damage)

Tail Grapple: creatures hit by the serpent's slam attack must make a DC 13 Dexterity throw. On a failure, a target takes 5 (2d8 + 3) bludgeoning damage. If it is Large or smaller, it is also grappled (escape DC 14). Until this grapple ends, the target is restrained and unable to breathe unless it can breathe water. If the saving throw is successful, the target is pushed out of the elemental's space.

The elemental can grapple one Large creature or smaller at one time. At the start of each of the elemental's turns, each target grappled by it takes 2 (1d8 + 4) bludgeoning damage. A creature within 5 feet of the elemental can pull a creature or object out of it by taking an action to make a (DC 13) Strength or Dexterity check and succeeding.

Undead Transport: The Sea Serpent can let out a fearsome roar that pushes creatures in a 15ft cone back 10ft on a failed DC: 10 Con save. While roaring, the sea serpent drops two zombies from it's mouth. The zombies act on the same turn as the sea serpent, this action can only be used once per battle.