Aquatic Athlete is a gear perk that improves your swim speed. It helps on Yara's many coastlines and river routes, and when escaping by water.
You move faster through deep water, which makes it easier to reach offshore targets, sneak around bases from the sea, or escape a fight by diving in.
Wear one of the gear pieces that carry it: Angler's Sandals, Poolside Flip Flops. Each piece only takes one body slot, so plan around the other perks you want.
Situational. Useful on coastal operations, but most of the time a vehicle or boat gets you there faster.