Neptune's Shell

It drops from the Creature from the Deep, which only appears during a solar eclipse, and an eclipse will not hand you a steady stream of them on its own. Eclipse enemies have no fixed spawn rate, so the fix is to force the issue: a Water Candle placed nearby and a Battle Potion running will push far more of them onto your screen than waiting quietly does. Fight on the surface with a cleared arena and the shell usually falls within an eclipse or two.

You turn into a merperson and the usual underwater penalties stop existing. Your breath stops draining, the sluggish movement goes away, and the jump key becomes a swim stroke that carries you at something close to flying speed. Ocean floors, flooded caverns and the Shimmer pool turn into places you travel through rather than places you survive. It works on honey too, though honey keeps its thickness and you still crawl.

• Breaking the surface and swimming upward does not refuel wings or rocket boots. A water column is not a free flight reset.
• The merfolk form overrides the werewolf one, so a Moon Charm does nothing for you while you are wet.
• In a vanity slot you get the appearance and none of the function, and your breath still runs out normally. That has drowned plenty of people who assumed the look implied the buff.
• Dyes on your armor slots color the merfolk form. The dye slot next to the accessory itself does nothing.

It covers everything Diving Gear does, but it is not a strict replacement for every version: you lose the ice traction of the Arctic Diving Gear and the underwater light of the Jellyfish Necklace. It also will not stack with its own upgrades. Long term it goes into the Moon Shell, which packs this and the moon transformation into one slot, so keep it rather than selling it.