How about the dolphin encounter in Cozumel? I understand there is no swimming involved with the encounter. They have you stand on a submerged platform, but you still get to interact with the dolphins. Hope you guys have a fantastic journey!
They are still fairly active, can get around well. But I don't think swimming will be an option for them. They do not know how to swim very well. Both are interested in history, and my grandmother enjoys scenery.