Neptune's Shell is a Hardmodeaccessory that aids performance underwater (and in other liquids) by granting indefinite breathing time underwater, swimming via the ↷ Jump key, and eliminating the slow movement speed usually caused by water. Entering water with this accessory equipped transforms the player into Merfolk form. While in this form, the breath meter will not display (nor will it decrease), therefore effectively preventing drowning.
The Merfolk effect takes priority over the benefits of Moon Charm and its upgrades, causing the player to lose Werewolf's bonuses while in water.
Neptune's Shell can also be equipped as a vanity accessory, which causes the Merfolk form to appear regardless of whether the player is in water. This is merely the appearance of the Merfolk, the buff will not take effect while it is equipped as a vanity (caution should be taken when in water, as the player will need to watch their breath meter).
Wearing a Fish Bowl will activate the Merfolk buff.
Half of the ingredients can be obtained pre-Hardmode, so it pays to collect them as early as possible.