Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Water Breathing. The sea serpent can breath only underwater.

Multiattack. The sea serpent makes two bite attacks and one tail attacks. 

Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 21 (3d8 + 8) piercing damage, and if the target is a creature, it must succeed on a DC 17 Constitution or be poisoned for 1 hour. While poisoned in this way, the creature is incapacitated. 

Tail. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 16 (6d8 + 8) bludgeoning damage.

Wave Pulse (Recharge 5-6). The sea serpent exhales a solid burst of water in a 60-foot cone. Each creature in that area must make a DC 21 Strength saving throw. On a failed save, a creature takes 62 (10d10 + 7) bludgeoning damage and it pushed to the end of the cone opposite of the sea serpent. On a successful save, a creature takes half as much damage and isn't pushed.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Detect. The sea serpent makes a Wisdom (Perception) check.

Tail Attack. The sea serpent makes a tail attack.

Thrash (Costs 2 Actions). The sea serpent flails around, smashing into the things around it. Each creature within 10 feet of the sea serpent must succeed on a DC 21 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The sea serpent can then swim up to half its movement speed.