Obsidifish

This is not a normal catch. It lives in lava and nowhere else, so the standard fishing kit is useless until you solve that first. The cheap route is baiting with critters caught in the Underworld, which works with whatever pole you already own. The permanent route is a lava-capable hook or the Hotline rod, and either one turns lava fishing into something you can do without thinking about it.

The Potion of Return is the entry that justifies the setup. A free trip back to where you were standing changes how you handle boss arenas and long mining runs, and it is worth keeping a stack around permanently. Inferno Potions are situational and the Seafood Dinner is just food.

Bites are rare even with the right gear, so bring plenty of bait and expect to sit by the pool for a while.