Underwater Adaptation is a gear perk that lets you hold your breath longer while swimming. It helps you explore underwater, reach submerged loot and escape enemies by diving.
You can stay underwater longer before you need air. That opens up deeper dives and longer escapes.
Wear the Gar Helmet. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Tough Scales protects you from aquatic predators while you are in the water.