The state of Tabasco on the Gulf of Mexico is suffering its worst flooding in more than half a century, with 1m people affected and at least one death.
Four-fifths of the state has been flooded and the waters are still rising, according to Red Cross officials.
The state governor has asked boat-owners to assist with the rescue efforts, alongside army and navy crews.
The floods have contaminated the state's clean water supply, according to Tabasco's ministry of health, and people are having to queue for safe water.
Shelters have been set up throughout the state, with some people gathering in churches.
The state capital, Villahermosa, is badly flooded after the Grijalva river burst its banks.
Weather forecasters are predicting that a new cold front could bring more rain over the weekend, and the state has been placed on high alert.