Then buy a movie camera or video camera and go out and shoot. Learn to critique and edit your own work. You'll have to be able to manage your money well, because some years there will be plenty and some years there won't be. Becoming a self-supporting underwater photographer or filmmaker is possible, but difficult. Being an underwater filmmaker is different than being an underwater videographer.
Filmmakers produce products, a videographer goes out and shoots something.
There are no jobs in this world for underwater filmmakers, you work for yourself and sell your shows. However, with more cable channels and more interest in the environment, there are more outlets for your work. There may be a few jobs for underwater videographers with oil companies or hull inspection services, with tourist operations, or even with research outfits. But not many and they may not be full time. I hope to be doing exactly what I'm doing now.