everybody offering great advice, JohnnyUtah hit the nail on the head. You have to train yourself to take a deep breath before going under, it becomes instinct after a while and you don't even think about it. Secondly, at that moment that your getting thrashed and pushed around, you have to train yourself to let go, go into a completely relaxed state, the more relaxed you are the more efficient you will be with holding your breath, because the power of the ocean is going to win every time. Try doing the breathing exercises as mentioned above, and even the freediving courses that Erock mentioned would def give you more confidence. Whoever is knocking Erock must not have a clue.