Emergency services found the woman, whose name has not been officially released, dead at the scene in Melaque Bay, Cihuatlán municipality, on Saturday, according to a statement from the local civil protection and fire service, posted on Facebook.
According to the statement, one of the victim’s legs was amputated, and the injury appeared to have been caused by a shark attack.
According to the statement, the accident occurred on the same day as a swimming race was held in the Gulf, which is located in the state of Jalisco, western Mexico.
A video widely circulated on social media, which CNN has not been able to verify, appears to show a person being carried from the water with one of his legs missing.
Local beaches will remain closed until further notice, the service said in a separate statement, adding that volunteers are patrolling to warn people to stay out of the water.
Pictures attached to the post show red warning flags in the sand.
The Cihuatlán municipal government also warned people to stay away from local beaches, in a statement posted on Facebook.
“Our priority is to ensure the safety and well-being of every citizen,” the statement read.
The nearby municipal government of La Huerta also warned citizens of “the presence of a shark on the southern coast of Jalisco” in a statement on Facebook.
“Travel specialist. Typical social media scholar. Friend of animals everywhere. Freelance zombie ninja. Twitter buff.”