Neptune's Shell

Neptune's Shell is a Hardmode Accessory that aids players while under Water. Entering water with this accessory equipped transforms the player into Merfolk. While in this form, the slowdown normally experienced while in water is negated, swimming is enabled via the jump key, and the player can remain underwater indefinitely without drowning.

Neptune's Shell can be considered an upgraded version of the Diving Gear in most respects, however the Diving Gear works in both Water and Lava (eg. while using an Obsidian Skin Potion), whereas Neptune's Shell only works in Water.