Underwater respirator

GILL

Fish rely on this to breathe while swimming through ocean currents.

An underwater respirator is something that lets creatures breathe while submerged, and in nature, fish use gills for exactly that. The clue plays it straight—no wordplay or tricks—just a direct definition pointing to the biological organ that extracts oxygen from water. If you thought of scuba gear first, that’s a man-made version, but the answer here is the natural one.