Flipper

With it equipped, jump keeps working underwater. Tap it repeatedly and you climb through liquid as though you had unlimited double jumps, which turns the ocean floor and flooded caves from a breath problem into ordinary travel. It applies to honey and lava as well, though lava will still cook you without separate protection.

It does not do everything the potion does. You still wade rather than walk, so horizontal movement in water stays sluggish, and none of the accessories built from it fix that either.

It feeds two separate accessory lines, the diving gear line and the frog line, and both end in things worth wearing far later than you would expect from a chest item this early. Keep the first one you pull.