I am CURRENTLY on Royal Caribbean cruising back home. This is my second dive with Doctor Dive in Costa Maya. They are an EXCELLENT outfit, they take great care of you and the diving is very good. As others have mentioned make sure you will have enough time to make the ship. So for example, my ship had a last call at 4pm. Based on that, I gave myself a 3pm limit, to pad for any unforeseen delays.
From the cruise terminal it's about a 10-15 minute taxi ride and it costs $8-$10 USD in cash. I'd advise to bring small bills as the taxi's may not have change. When you are initially dropped off, it's in a small neighborhood, you think you're in the wrong place but then as you walk it opens up to the beach and it's very nice. There are places to lounge around and there are restaurants and bars along the beach.
Getting back to Doctor Dive, have no worries, they will take good care of you. Charlie is very good with emails and communicating and you will have time to get in a two tank dive with time to spare.
Oh one more thing, they also offer photo and video services, so you can have a nice momento of your dives!