Th second largest coral reef system in the world starts in Cozumel and ends in Roatan, so there should be plenty of scuba and snorkeling opportunities at all the Sun's ports. I suggest going to cruise critic's port forums to get recommendations for different vendors/tours.
There's much to do on a Western Caribbean ashore, so take your time and pick the excursion you wish to do.
The only excursion I will recommend is in Belize, the New River Boat tour to Lamani. Although it rained heard on my trip, it was certainly worth the money.
Have a great cruise.