<<Our ports are Grand cayman (maybe) Roatan, Belize and Cozumel.>>
In Grand Cayman, take a taxi to Seagrape Beach and then walk south or north for snorkeling, right in front for great swimming. Or, consider doing a Sting Ray City Tour if you haven't done it.
In Roatan, don't waste a second. Take a taxi to the Half Moon Bay Inn on the West Bay for great snorkeling and swimming. Walk into town for shopping and more.
In Belize, either fly over to Ambergris Cay for the day or take one of the ship's snorkeling tours.
In Cozumel, try Playa Palancar for great swimming and then negotiate for a boat to take you to Palancar reef for snorkeling.
Check out www.CaribbeanPortReviews.com
for tons of more information or check out the CruiseReviews.com snorkeling guide prepared by Tom Kanitra at: http://www.cruisereviews.com/info/snorkeling.htm
Have a great cruise.