The Straw Hats breathe in Fish-Man Island because the island itself has huge air bubbles and enclosed air-filled spaces, so the humans aboard can breathe normally while underwater. Fish-men and merfolk are also amphibious, meaning they can breathe underwater and on land.

Quick Scoop

Fish-Man Island sits inside a giant bubble structure deep under the sea, and that bubble contains air pockets large enough for human visitors to survive there.

Why the Straw Hats can breathe

  • The island is not just open ocean; it has an air-filled bubble environment.
  • Human crew members use that trapped air just like they would on the surface.
  • Fish-men themselves are different: they have both gills and lungs, so they can switch between water and air breathing.

Simple version

Think of it like an underwater city wrapped in a giant air dome. The Straw Hats are breathing the air inside the dome, not the surrounding sea.

Tiny caveat

If they were outside the bubble protection, humans would obviously not be able to breathe underwater for long. The setting works because Fish-Man Island’s geography and bubble systems make the interior habitable.

TL;DR: They breathe because Fish-Man Island has air bubbles and enclosed air pockets, while fish-men can breathe both underwater and on land.