A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles. The particles can also be biological in origin, such as mollusc shells or coralline algae.
Swimming is good and it is mentioned in Hadeeths encouraging us to teach our sons swimming among other useful skills, but it should not happen in a place where women are swimming with indecent cloth.
Swimming in such place can cause sinful act of looking at women with indecent cloth.