From a natural history perspective, the underwater ecosystem is actually very accessible. You can have close encounters with wildlife, even with big animals, and many of the relationships between animals are easily observable -- commensalism, which includes symbiosis and parasitism, is literally everywhere.
I love learning about natural history, and it's as important to me is the photography itself.
On land, it can be very hard to see wildlife up close, and if you do happen to get close to a big wild animal, it is often dangerous.
People say they wish they could be doing what I'm doing. I do feel extremely fortunate that I get to do what I do, but I also feel like almost anyone can get into photography if they spend enough time doing it.
We live in a time of unprecedented access to camera equipment--there are literally billions of cameras on the planet--more cameras, in fact, than there are people. Interesting subjects are also almost everywhere on the planet, and subjects become interesting when a compelling stories are created about them and shared.
The last pieces are having an audience, and most importantly, spending a lot of time behind a camera. Audiences are easy these days because of the Internet, which leaves time spent taking pictures... which takes me back to what goes through my mind when someone says that they wish they could do what I do. If you spend thousands of hours doing something, you will probably become an expert, especially you are naturally predisposed to being a storyteller. It's really simple: go out and spend all of your free time capturing and telling stories about something you care about, and after a year or two, you will suddenly find that you have an interesting body of work. At this point, you'll also probably discover that you are very knowledgeable in both photography and in whatever domain you have chosen as your subject of choice. Good luck!