Source: Tal'Dorei Campaign Setting Reborn

Weapon (any ranged or thrown), rare (requires attunement)

This elegant weapon is adorned with engravings depicting leaping dolphins and drifting jellyfish. You gain a +1 bonus to attack and damage rolls made with this magic weapon, and you ignore the normal penalties on ranged weapon attacks while using this weapon underwater. While holding the weapon in front of you with both hands, you have a swimming speed of 30 feet.

Additionally, while you hold the weapon, you can use an action to cast the Water Breathing spell. Once this property is used, it can’t be used again until the next dawn.