Blood Frenzy. The warlock has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Limited Amphibiousness. The warlock can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating.

Shark Telepathy. The warlock can magically command any shark within 120 feet of it, using a limited telepathy.

Innate Spellcasting. The warlock's innate spellcasting ability is Charisma (spell save DC 13, +5 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: eldritch blast (see "Actions" below), minor illusion

1/day each: armor of Agathys, arms of Hadar, counterspell, crown of madness, invisibility, hunger of Hadar

Multiattack. The warlock makes two attacks: one with its bite and one with its Sword of Fathoms.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit:4 (1d4 + 2) piercing damage.

Sword of Fathoms. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit:7 (1d10 + 2) slashing damage, and if the target is a creature, it must succeed on a DC 13 Constitution saving throw or begin choking. The choking creature is incapacitated until the end of its next turn, when the effect ends on it.

Eldritch Blast (Cantrip). Ranged Spell Attack: +5 to hit, range 120 ft., one creature. Hit:5 (1d10) force damage.