Echolocation. The whale can't use its blindsight while deafened.

Hold Breath. The whale can hold its breath for 30 minutes.

Keen Hearing. The whale has advantage on Wisdom (Perception) checks that rely on hearing.

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

Multiattack. The Undead Killer Whale makes one attack with its bite and one attack with its tail per turn.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 4) piercing damage plus 1d6 necrotic damage. In addition, any creature hit with their bite must make a constitution saving throw or be "infected" - repeating the 1d6 necrotic damage at the start of the creature's turn and giving them disadvantage on their next attack roll as if they had been poisoned.

Tail. Melee Weapon Attack: +6 to hit. reach 10 ft., one target. Hit: 2d6+2 Bludgeoning Damage. On a hit the target creature and all those within 5 ft of it must make a Dexterity saving throw of 13. If they fail, they are plunged 15 feet under the water by the impact of its tail or the wave it creates.

Rock the Boat: As a bonus action, the whale can ram head first into the side of a boat to attempt to knock sailors off of it. It makes a strength check against their dex to see if they fall off (granted they were within 10 ft of the edge of the ship).