Honestly, the biggest piece of advice I can offer is perseverence. Keep trying. Push yourself into an environment or situation that pushes the envelope of your comfort. Once you convince yourself you are okay and all's well, and you begin to get comfortable at that point, push yourself further yet again.
Try the pool. Shallow end, deep end. Maybe just snorkling and diving to the bottom and back. Then maybe move on to scuba. Just sitting in the shallows, relaxing and breathing. And so on.