We just got back from a (non-Disney) vacation on Grand Cayman. I thought 7-mile beach was OK, but Rum Point was just fantastic! It was the only place we went to twice on our vacation. It is just such a neat scene. Lots of hammocks, lots of shade, even the water is shaded! It is perfect for families with small kids. The water was completely flat. We also did stingray city from Rum Point. I don't know how the cruise excursion works, or how much time you have etc., but Rum Point is one of the best places on Grand Cayman (in my opinion, of course!). If you don't want to do an excursion, the best snorkeling on all of Grand Cayman is right near where your boat will anchor! We went in the water at the Eden Rock Dive shop, swam towards the cruise ship, and the snorkeling was amazing. (Note: this is not on 7-mile beach, it is literally just landward of where the ship is anchored).
Hope this helps, Maggie