Here are some things to keep in mind about a career in u/w cinematography.
1. You are talking about a very small slice of a very specialized pie. Take a look around and ask yourself how many shows or documentaries that are on TV are about the u/w world? Not Many. And with the fragmentation of the media into broadcast, online, cable, etc. the budgets get smaller while the quality demands grow.
2. On these u/w shows or docs how much of the show is actually underwater? At best less than 50%. The rest of the story is shot above the water.
3. Because most u/w shows aren't actually underwater you need camera, lighting, audio, producing, writing and editing skills. You need to have a proficiency in at least a couple of the above skills and an understanding of all of them.
4. You must be an awesome/experienced diver and shooter. The u/w world is unforgiving and if you don't know how to stay alive at depth AND make great pictures then you won't do either well. These skills are earned with time not bought in a class.
5. You also need to know something about business because in the end bean counters make the decision about whether to fund you and your project. Great ideas are a dime a dozen selling them is the real skill. In my best years only about 20% of my income has come from u/w work. The rest of my work involves shooting footage for terrestrial clients.
6. Finally, build yourself a solid skill base. There are lots of ways to get to your goal. Most of them are probably ok but there aren't any shortcuts. Find your niche and own it.
Best of luck!