We took my in-laws when they were 79. Motherin law took a "ski belt" as she was use to using this while floating in the lake. If possible, take a snorkeling class/instructuction before your trip to learn how to clear mask. Snorkeling is super fun and does not require being a good swimmer. As stated above, snorkel with a noodle as it gives you security and also makes you more visible / and easier to be spotted by your group if many people in the water. Can take dingy to everywhere ... at the Baths, there is a significant swim to shore, but can swim the rope. We prefer to do the baths as a land trip, visiting from The Top Of The Bathes... And due to path thru boulders, may want to just view from boat.
But BVI and sail trip was very memorable to in-laws, and just in time as now their agility (and health) has gone down significantly. GO!
