In the Bahamas we took one of those excursions to go diving to see the coral beneath. The captain/owner looked like he loved what he did showing us everything underwater (hand signals) but also looked pretty stressed because all their income comes from tourists. If you take away that 1 factor, I'm sure it would be a great gig.
Yeah generally speaking, they do not make much money and don't live any kind of lavish lifestyles or anything. I know a lot of dive masters don't even get paid they work only on tips, but obviously they get to do dives for free while guiding people. Most of them also get free air fills and tank rentals from the companies so they can dive for free on their spare time though.
But I was talking with the dive masters about this when I was getting certified and they pretty much work 7 days a week from 7am to 4 or 5pm. And most of them will move to a country for a year or two then move to another country and work for a dive shop there. The one I was talking to lived with 3 other people who all work in the industry in a small house. It's just a certain lifestyle.
That said though, that's still a bit different than a captain. I would do more of a private charter type of gig instead of these mainstream ones. But ... it's never going to happen because it's just not in my cards.