Become an expert diver. A camera in your hand makes mask clearing, gauge checking, and BC inflation much more complicated. Cameras tangle in kelp. You can't master scuba photography until you've mastered diving. Learn the science.
Good photographers know how to get the picture.
Sometimes your dive store or dive boat can rent you a very nice camera for very little money. But once you decide photography is for you, buy. As with your scuba equipment, it's safer and more satisfying to work with your own camera gear. If you're on a tight budget, understand what the limitations of your cheap equipment are, and work within them. If your cheap camera will take pictures only under ideal conditions, and only with subjects of a certain size, become an expert at exactly that type of picture.