Gaviotas beach lifeguard Lazaro Magana says the victim was taken to the hospital Thursday with an 8-inch (20-centimeter) wound in her foot.
Magana says the woman ventured 100 yards (meters) into the water even though he warned swimmers to stay away because of the possible presence of sharks.
Police say the woman is from Russia but haven't released her name or hometown.
It was the second shark attack in the hotel zone of Cancun in less than two months. A Canadian tourist was seriously injured in early