If you want to get into UW film making especially doing documentary as a career, it would take more than that. I would think you need some sort of Marine Biologist background, etc. The well known ScubaZoo in Borneo require that kind of background to even be an intern.
Regardless, you should start by getting this DVD and understand the career path:
I have a friend who just turn 20. He wants to be a UW film maker, now attending film school, then decide to become a pilot because independent film maker doesn't make much money and cost a lot when come to equipment. He think flying as a pilot at least give him a chance to go diving at different location. Go figure.