For what it's worth, my advice would be to carry on doing what you are doing, trying to relax as you swim and trying to breathe easily. The main secret of breathing in swimming, I believe is to breathe out while your face is in the water. Stay in the shallow water for the time being and work on your relaxation and breathing. It isn't possible to relax properly if you are worried or afraid. It probably won't be long before you find yourself more at home in the ocean, but keep an eye out for some of that aquatic wildlife you have in those parts!