Roatan: Both Tabyana Beach and Maya Key have great snorkeling. DW and I prefer the latter because it's a shorter ride from - and to - the pier.
St Maarten - America's Cup Regatta. Two ships race against each other. You can do nothing, or actually participate in working the boat. Lots of fun, and I believe this is offered in other ports.
St. Thomas - lots of good options for snorkeling. Powercat Snorkel is a very nice excursion. Magen's Bay is a beautiful beach, but it gets crowded. DW and I are early risers; we go there in the first group, spend a few nice hours, then head back.
Tortola - just take a cab to Cane Garden Bay. String of small bars/inns, and for a small fee you rent a lounge on a gorgeous beach. Another option would be shore excursion to Jost Van Dyke. Beach at Soggy Dollar Bar is very nice, and this place claims to be the inventor of the Painkiller drink.
Cozumel - they offer America's Cup Regatta here, as well. Pretty much any of the beach offerings are nice. Playa Mia is popular, near the port, and has some easy and nice snorkeling. It gets crowded. A great excursion is Amazing Secret River. No hiking, but you do need to have a little "dexterity" as you are in an underground cave, and sometimes wade in the river with shallow head room. My wife has back issues and did this with no problem.