Depends on where you are talking about - Nassau or Castaway Cay?
There are several options on Nassau: book an excursion that includes beach access or go to Cable Beach which is a public beach.
On Castaway Cay, you have the Family Beach, the Sports Beach, the Teen Beach and Serenity Bay (for Adults only). There are other parts of the beach that are just for boating or snorkeling, but there is plenty of beach for just swimming.
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