In Cozumel - our dive shop operator goes there 5 or 6 times a year. He says Chakanaab is the best snorkeling. We had our parasailing cancelled in Labadee so we rescheduled it for Cozumel. It rained us out in Cozumel - so we didn't get to parasail or snorkel. Our friends did the dolphin experience through the line in Cozumel and really enjoyed it.
In Grand Cayman - Don't miss Stingray City - great experience.
Ocho Rios - On the western route but you didn't mention it... Highly recommend Peat Taylor. He will take you wherever you want to go and is very honest, professional, dependable, and downright enjoyable!
Key West - Mallory Square and then ask the locals where they eat. I had the best conch fritters, coconut ice cream, and key lime pie becasue I didn't go to a tourist spot but a hole in the wall local favorite. Go for it!