The "s" in the acronym "scuba"

SELF

Think of what the diver carries on their back—this word describes the source of air that doesn’t rely on the surface.

The acronym "scuba" stands for "self-contained underwater breathing apparatus," so each letter represents a word. The clue asks specifically for the word that the letter "s" stands for in that acronym. That word is "self," making it the straightforward answer to the puzzle. No wordplay here—just a direct reference to the acronym's meaning.