Modesty in a bathing suit

Q: It is permissible for a young woman to wear a bikini at a separate all-women's beach?
A: If there are only women, there is no problem and any bathing suit is permissible. If there is a male lifeguard at the women's beach, which is permissible since it is a potentially life-threatening situation, there is a problem and she must wear a robe until she enters the water.