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Don't bother with a cruise line tour if you're just going to the beach (unless time is of concern, such as it's your last day there and you have to be back to the ship before it departs). It's far less expensive to go on your own, and not that difficult. There are public buses that leave from the cruise ship dock, and at least one of these goes by the south shore beaches. You'll hear Horseshoe Bay mentioned a lot -- it's popular with tourists, and can get a bit pricey once you add in snacks, chair rentals, etc.
My favorite beach in Bermuda is Shelly Bay Beach, located near the Bermuda Aquarium. It's a small beach with calm, shallow water. I don't recall many amenities last time we were there, though. And, with the Aquarium so close, it can be a great day trip. To get there, you'd take the bus to Hamilton, where you will transfer to a second bus. It's not as difficult as it sounds -- the bus drivers are extremely helpful.