Call Me Ishmael. If the Sea Serpent moves at least 20 feet straight toward a nonliving target and then hits it with a ram attack on the same turn, the target takes an extra 23 (4d8+5) bludgeoning damage. If any creatures are standing on the nonliving target they must succeed a Dexterity (Acrobatics) check (DC 16) to avoid being knocked prone. If this is a ship, the captain then makes a Vehicle (Water) check with DC equal to half of the damage taken. If the captain fails, then the ship's movement speed is dropped to 0 the following turn.
Hold It Steady. If hit by a weapon that is ship mounted, double the amount of damage dice for that attack done to this creature.
Elusive Little Beastie. If submerged in water at least 30 feet deep, this creature gains advantage on Dexterity (Stealth) checks.
Echolocation. While submerged, this creature gains it's Blindsight unless it is Deafened. If deafened, this creature will not submerge deeper than 30 feet.
Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 21 (3d10 + 4) piercing damage. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the Sea Serpent can't bite another target. On it's turn, if the Sea Serpent is currently grappling a creature, it may use it's Swallow attack.
Ram. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage.
Swallow. The Sea Serpent makes one bite attack against a Large or smaller target it is grappling. If the attack hits, the target is instead swallowed and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the serpent, and it takes 20 (4d10) acid damage at the start of each of the serpent’s turns. The serpent can have only one target swallowed at a time.
If the serpent takes 15 or more damage on a single turn from the swallowed creature, the serpent must succeed on a DC 14 Constitution Saving Throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the serpent. If the serpent dies, a swallowed creature is no longer restrained by it and it can escape from the corpse by using 20 feet of movement, exiting prone.
Hurl. The Sea Serpent may use it's action, while grappling a creature, to hurl it rather than swallow it. The creature is then flung in a direction of the DM's choosing for 30 feet. If the creature strikes an object, they take (1d8+6) bludgeoning damage for each 10 feet they were flung before hitting an object. If they travel the full 30 feet, they then take fall damage equal to how high they fall from. They may make a Dexterity Saving Throw (DC 16) to right themselves before impact to take half-damage. If failed, the creature is prone and stunned until the end of it's next turn.
Deadly Fall. If the Sea Serpent jumps at least 15 feet as part of its movement, it can then use this action to land on its feet in a space that contains one or more other creatures. Each of those creatures must succeed on a DC 16 Strength or Dexterity Saving Throw (target's choice) or be knocked prone and take 14 (3d6 + 4) bludgeoning damage. On a successful save, the creature takes only half the damage, isn't knocked prone, and is pushed 10 feet out of the Sea Serpent's space into an unoccupied space of the creature's choice. If no unoccupied space is within range, the creature instead falls prone in the Sea Serpent space and is considered stunned until the Sea Serpent moves from on top of them. They cannot be attacked by the Sea Serpent if underneath it.
The Sea Serpent, if used as a Legendary Creature, has these following Legendary Actions. The Sea Serpent has a total of 2 Legendary Actions.
A Swish of the Tail (Costs 1 Action). The Sea Serpent may move it's full movement speed without prompting opportunity attacks.
It Came From The Blue (Costs 2 Actions). The Sea Serpent may use it's Deadly Fall attack if it did not use it the previous turn, and is not grappling a creature.
And It Was Gone... (Costs 2 Actions). The Sea Serpent may use it's full movement speed away from the closest target, and attempt to make a Dexterity (Stealth) check. If it is below 1/4 of it's Max HP, on it's next turn the Sea Serpent will attempt to escape.