hi laura, first you didnt say if you knew the shore excursion from RCCL if not heres there sitehttp://www.royalcaribbean.com/asp/default.asp then go to excursions then click caribbean ,then pick the islands your ship is visiting , i think the two ports for snorkeling that you have to choose from is cozumel and grand cayman, but in cayman you have to go swim with the stingrays if you never have done it before,in jamaica there's dunn's river and labadee is RCCL privite island so beach and BBQ and its a relaxing day .so i think cuzumel is your snorking port .
You wil want to go to cozumel for snokling. It has the second largest reef in the world. We just came back last Nov. where we did an all day trip of snokling, speed boat and atv's. Loved the snokling and was scared to death on speed boat because the water was choppy. The atv's were not that great either. Book thru RCI or online at shoretrips.com
There are also some great sites BESIDES Stingray City around Grand Cayman. A place called The Gardens and some other spots. When we were there last, we had arranged ahead of time via the internet to go on a tour that was NOT booked through RCCL and were taken to 3 sites, one of which was Stingray City. (Think I found the site by just typing in Grand Cayman on Ask Jeeves or maybe just www.caymanislands.com?) I think we paid about $30 or $40 pp for this tour which took 3 hrs. We had to go directly to the office about a 5 min. walk from the main docks to get to the place we were to meet. Also, there are independent guides right near the docks in town who are hawking their tours at VERY reasonable prices. There is an info. booth type thing right near there, too, which you will see. They can also arrange tours. Having said that, we have snorkeled in Cozumel several times and enjoyed it a LOT. A couple of times from catamarans and the last time from Chankanaab Park, just a few miles from town and not expensive. That was GREAT snorkeling! Though you shouldn't miss Dunn's River Falls, the area around Ocho Rios isn't all that great for snorkeling. Do Dunn's and save the snorkeling for other locations. Hope this helps!