I'm a reasonably experienced diver (Advanced Open Water certified) and have been diving all over the world. Like many people who like diving but aren't made of money, I've figured out that a cruise is an easy, cheap way to get a vacation that also includes some diving. But when you book a dive through your cruise ship, sometimes it is ok, sometimes not. Sometimes you get a shop where things are a bit too casual, or the equipment is substandard/worn out, or the guides are lazy and don't do much more than cut you loose to explore on your own. This place is none of those things. The shop is well-run, the equipment is fine, the guides are friendly and knowledgeable. You will feel very safe diving with these people. In fact, we actually had a guy on our dive who brought his own regulator and his mouthpiece came completely off his regulator under water (I'd never seen a worse malfunction, ever). The guide was on him almost instantly and got him back under control before I'd even realized what the problem was. So they do know what they are doing. They break you up into reasonably small groups on small boats, so there is none of that cattle call feeling when you are on a single, big boat. We were a group of just 4 people and a guide, and I think 6 or 8 was about as many as they put in each boat.
It is located in the middle of absolutely nowhere on the Mexican coast, and the people there appear to be uniformly grateful that you've come to dive with them, as it is a huge part of the local economy. So they bend over backwards to treat you right so you will write good reviews on TripAdvisor like I'm doing now.
They do have lion fish (we saw one), but the reef is still looking pretty healthy. We saw a nice variety of marine life, the highlights included a huge lobster, several conch, and two big sea turtles (one of which swam by maybe 8 feet from me). This is all part of the huge reef system that includes Belize, so if you have ever been diving in Belize before, this will feel pretty familiar. You don't have to go miles off shore to get to the reefs, the depths are moderate so you get decent bottom time, etc.
We came off a Holland America ship, but there was also a group off of Carnival. I think this is the dive operator that caters to all the cruise ships that stop at Costa Maya, so if you are coming there on a cruise ship, this will be your dive operator.
Probably the only criticism I could possibly level at the place is that there aren't enough bathrooms in the shop given how many people they serve at once. Particularly if you want to change after you go diving, one bathroom for the whole shop is a little sketch, even though this is rural Mexico. They could at least offer some places to change after the dive. But that is a pretty minor little nit. You should feel no hesitation about booking a dive with these guys if your ship stops in Costa Maya.