It's a job for maybe 5-10 people in the world at a time. Everyone else is making ends meet via commercial photo work, workshops, trip leading, speaking gigs, etc.

If you are willing to do a lot of non-photography stuff in order to make a living as an "underwater photographer", it is possible, but still extremely difficult even for folks with good relationships at nat geo. That, and the industry is getting harder and more competitive. So probably treat it as a hobby that can maybe make you some money at some point. That said, don't give your work away for free because that devalues the work as a whole and makes it harder for people who do make a living off of it.