When you are in the water, hold L2 + R2. When your player surfaces, keep holding the buttons. Your character should go back underwater, except deeper this time, allowing you to stay under longer. If you come up to a wave, press L1 + L2 to go under it. Hold L1 + L2 until you surface. You will go underwater all the way to the bottom. This gives you a chance to stay under longer and look around.