Instructions
Turn your character to face the body of water you want to enter, using the right analog stick on your PS3 controller.
Move your character into the water with the left analog stick. Your character can walk through shallow water, but will begin swimming when the water reaches approximately chest depth. A character in Stealth mode begins swimming sooner because he's in a crouched position. To toggle Stealth mode, press the L3 button on your PS3 controller.
Pull back slightly on the right analog stick to look "up." This keeps your character's head above water. To swim underwater, push forward on the right analog stick to look down and dive deeper. When your character is swimming underwater, a transparent "O2" indicator gauge appears in the middle of your screen.
Move your character through the water using standard movement controls. The right analog stick controls direction, while the left analog stick controls forward or backward movement. Remember that looking down will cause your character to dive underwater.
Watch the "O2" indicator carefully when swimming underwater. It indicates when your character is running out of air. If you don't surface for a breath before the indicator bar completely disappears, your character will take damage from the lack of oxygen. Note that the time the indicator takes to count down is directly related to your character's Endurance (EN) statistic; the higher the Endurance, the longer you can stay underwater.