My wife and I went on Mariner at the end of September this year to the Western Caribbean. I agree with the other poster here that Ocho Rios was our least favorite port-we basically stayed onboard. However, I agree that you'll like Georgetown, Grand Cayman and do go to StingRay City if you can. We booked the trip on the RCCL website to go to Stingray City. I've heard that there are other ways to book the trip with sites other-than on RCCL; some may be cheaper and handier, but being an ex-cop, I tend to not trust people or places I don't know well, and tend to take chances with places I can get back to easily if something goes wrong.
Ocho Rios I'm sure has some wonderful places to go and see, but this web site had some bad things to say, and I think you can trust everyone who posts here, and who have done the trips before.
If you go, you'll like Labadee, Georgetown, and probably Cozumel; expect to be almost run over by the junk shop workers any where you go, including Labadee. If you can be firm and brush them off, good for you. I can't, and tend to get mad-I'm not on a cruise to get mad, so I avoid them totally.
As for the Mariner, you will LOVE the ship and everything to do upon it-see shows, gamble, eat, swim, rest,spa, etc., and have fun!
Don't buy the pop-drink stamp. It's a scam as far as I'm concerned-that's just my opinion.