Definetly go with Captian Marvins in Cayman Island. We went on a snorkle/stingray tour with his company and it was just wonderful. When we arrived at the spot for the stingrays our group was maybe 20 people and the waters were not very crowded at all. When we got ready to leave all the tours from the cruise ship were arriving and there was 50-60 people in each group, so as you can imagine it was very crowded. In Jamiaca please go with a tour that is offered with the boat. I found it to be extremely unsafe. We made the mistake of walking into town and were harrased. We did the party boat to Dunn River Falls. It was great. In Cozumel we booked a jeep tour privately with a company that was recommended to me. It was so much fun and a lot less expensive than the boat tour. http://www.cozumel.travelnotes.cc/tours/jeep.html
The website is above, go check it out. Have a wonderful time. You can email me if you have any questions.