Sharkbite Cleaver

Weapon (handaxe), uncommon (requires attunement)

The head of this handaxe is sharpened to resemble a shark and its tooth. Sharkskin leather is wrapped around the base of the blade. While attuned to the axe, you gain a swimming speed of 30 feet and can breathe underwater. When you make a ranged weapon attack with this axe against a target that's missing any of its hit points, the attack roll is made with advantage. On a hit, it deals an extra 1d4 slashing damage. Your weapon attacks with the axe suffer none of the penalties of underwater combat, and if you miss with a ranged attack with it, the weapon returns to you immediately after the attack.

There's blood in the water. Strike fast. Hit hard. Bite deep.