The collar is the point

A polo does something a rash guard cannot: it protects the back of your neck. That strip of skin between your hairline and your collar burns on every boat day and is the first place instructors and divemasters see damage after a season of work. Turn the collar up on the crossing, turn it down at the restaurant afterwards. The long sleeve and quick-dry fabric mean it doubles as a working layer for anyone spending full days on deck. For diving itself, layer a men's dive rashguard underneath.