I would not leave cameras, wallets, etc., etc., unguarded while swimming, or at any other time. You are asking to have them stolen. I'm not just speaking of the beaches in the DR; I don't know a beach anywhere in the world where you would be guaranteed that your unguarded possessions would still by lying on the blanket when you return from the water. If you don't have someone who can watch your belongings (another guest on the same beach, for example), then chose between playing in the water together while they are stolen, or take turns swimming.
I can't believe that you'll find anyone on this forum that will assure you that your untended valuables will be ignored by potential thieves that frequent beaches looking for careless bathers.