You don't have to pay Atlantis to go to the beach. Cabbage beach is on the right side of Atlantis and is free. It is not a great beach for snorkeling though. If you decide to go take a regular taxi, NOT the water taxi! Blue Lagoon is not on the island, you have to take an excursion or tour over to the little island. It is pretty nice and better for wading and snorkeling.
Hopefully someone will have some recommendations, I have only been to those two places.