The tourist ship left the port of San Felipe on the Sea of Cortez, a haven for windsurfers and sports fishermen, on Saturday but was struck by an electrical storm and capsized the next day, port officials and the navy said.
Five Mexicans and sixteen Americans were rescued by boats and a navy helicopter when the captain made an emergency call for help. The boat was found around 87 miles south of San Felipe.
"We are still looking for the rest of the people. The weather is really bad right now, with strong wind," said local port official Felipe Vallecillo.
Vallecillo said weather conditions were normal when the boat set sail on Saturday.