When you are in St Georges there are seeveral beaches that are frequented. If you go up to Tobacco Bay you will be joined by 1000 fellow cruisers. If you go over by Ft St Catherines, there is a very nice beach next to it with facilities, St Catherines beach.
You can walk far enough down to get away from the small crowd. However, just on the other side of the Fort and down a small path is Achilles Bay. I have never seen more than 10 or 12people on this beach. Snorkelling is good and depending on the tides you can walk between the rocks . If you require "facilities" it is a two minute walk around the fort to St Catherines beach. There is a private Club on the rocks above Achilles Bay.
The members act like this is their private beach , but, it is PUBLIC.
When you are in Hamilton, go to Horseshoe Bay, yes it is crowded , but, you can walk WAY down the beach and avoid the crowd, or keep walking and go to Warwick Long Bay or any of the other beaches along this stretch.
If you like a nice snorkelling tour, seek out Jessie James Cruises in Hamilton . They run a fabulous 3 stop, 4 to 5 hour tour.