Water Walking Potion

The tooltip undersells it. Every liquid counts, lava included, so this doubles as a way to cross the Underworld without an Obsidian Skin Potion. The catch is depth. Land hard enough on a lava lake and you punch through the surface film for a moment, and that moment is enough to burn. Descend deliberately with the down key instead of free-falling onto anything hot.

The Ocean is the obvious use. Turn the sea surface into a flat running lane and Duke Fishron or a Blood Moon Dreadnautilus becomes a normal fight rather than a swimming contest. It also kills fall damage onto any liquid, so deep water doubles as a free landing pad while exploring.

Lava Waders and Terraspark Boots do part of this permanently, at the cost of an accessory slot. If your build cannot spare one, the potion route is cheaper than it looks. The ingredients are safer to gather than Underworld herbs, just slower to stockpile.