Neptune's Shell

Neptune's Shell is a Hardmode Accessory that aids underwater performance by allowing indefinite breathing underwater, enabling swimming via the key, and eliminating the usual slow movement speed caused by water.

Entering water with this accessory equipped transforms the player into Merfolk, a buff with no time limit. While in this form, Water's usual effects are all eliminated. The breath meter is eliminated entirely, and movement through water is as quick as it would normally be on dry land. Holding the key also makes the character swim (fly continuously upward) in water, while falling is just as fast as falling through the air.

Neptune's Shell can be considered an upgrade from the Diving Gear in most respects, but lacks the underwater lighting of the Jellyfish Diving Gear and the extra mobility on ice of the Arctic Diving Gear.

Trivia

 * Neptune is the Roman god of the sea.