Pack Tactics. The shark has advantage on an attack roll against a creature if at least one of the shark's allies is within 5 feet of the creature and the ally isn't incapacitated.

Water Breathing. The shark can breathe only underwater.

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage.