Hold Breath. The Moasasaurus can hold its breath for 1 hour.

Brute. A melee weapon deals one extra die of its damage when the mosasaur hits with it (included in the attack).

Rampage. When the mosasaur reduces a creature to 0 hit points with a melee attack on its turn, the mosasaurus can take a bonus action to move up to half its speed and make a bite attack.

Relentless (Recharges after a Short or Long Rest). If the mosasaur takes 28 damage or less that would reduce it to 0 hit point, it is reduced to 1 hit point instead.

Siege Monster. The mosasaur does double damage to objects and structures.

Underwater Camouflage. The mosasaur has advantage on dexterity (stealth) checks while underwater.

Infect. The predatory marine reptile can infect prehistoric marine reptiles and turn them into zombies 

Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 14 (3d6 + 4) piercing damage.

Swallow. The mosasaur makes a bite attack against a Large or smaller creature it has grappled and the attack hits, the target is swallowed and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the mosasaur, and it takes 28 (8d6) acid damage at the start of each of the mosasaur's turns. A mosasaur can have only one creature swallowed at a time.
If the mosasaur takes 95 damage or more on a single turn from the swallowed creature, the mosasaur must succeed on a DC 17 constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the mosasaur. If the mosasaur dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 33 (4d12 + 7) bludgeoning damage.

 Undead Nature. A Moasasaurus zombie doesn't require air, food, drink, or sleep.