Flipper Potion

The obvious half is water: you swim, you keep your speed, and you do not spend an accessory slot on the flipper to do it. The better half is that lava and honey stop dragging on you as well. A lava lake you would normally bridge over becomes something you cross directly, and the jungle honey pockets stop turning a descent into a crawl. It does nothing about the damage, so obsidian skin or the right accessory still has to cover that part.

Digging a hellevator is the standout case, since the water pockets and honey are what turn a straight shaft into a slog. Worth checking your pots and crates before you brew, because these turn up in loot often enough that most players never make one on purpose.