Helm of the Slashing Fin

Wondrous item, rare (requires attunement)

While wearing this helm, you can speak its command word as an action to gain the ability to breathe underwater, but you lose the ability to breathe air. You can speak its command word again or remove the cap as an action to reverse this effect.

You can use the attack action to make an attack with the fin topping the helm, which deals damage as a great axe. You are proficient with the slashing fin. If you move at least 20 feet straight toward a target and hit it with the fin on the same turn, the hit is a critical hit.