Gaviotas beach lifeguard Lazaro Magana said the victim was taken to the hospital Thursday with an 8-inch wound in her foot.
The woman ventured 100 yards into the water even though he warned swimmers to stay away because of the possible presence of sharks, Magana told The Associated Press.
Police said the woman is from Russia but haven't released her name or hometown.
'Sea of red' It was the second shark attack in the hotel zone of Cancun in less than two months. A Canadian tourist was seriously injured in January.
"I just remember the water filling with my own blood," victim Nicole Moore, 38, later told reporters. "It was just completely a sea of red."
Moore lost her arm in the attack, the CBC reported. The nurse said she had waded into waist-deep water to wash sand off her feet after playing volleyball on the beach.