Throughout your adventure in Assassin's Creed Black Flag Resynced, there will be plenty of times when you'll find yourself underwater. One of the new features in this remake is the ability to dive in any body of water, rather than only at specific locations such as shipwrecks. Alongside this new feature, there's also an option to enable unlimited oxygen, so you only need to worry about dangerous animals rather than running out of air.
Enabling unlimited oxygen is very easy, and you can do it right from the beginning of your playthrough without having to unlock any special equipment or mechanics. Simply open the menu and go to System > Options > Gameplay > Play Style > Unlimited Oxygen.
That's the option you're looking for: check the box and Edward will no longer suffocate while exploring underwater. You can turn this option on or off at any time.
