The 115-foot (35 meter) chartered boat 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 early on Sunday, port and navy officials said.
Emergency services officials in San Felipe said 27 of the 44 people on board were U.S. citizens.
The U.S. Coast Guard, which is sending a helicopter to help with the rescue, said one person was dead and authorities were still searching for six others, but had no information on their nationalities.
"(The helicopter) should be there around 0800 (Pacific time) to assist with rescue efforts," Coast Guard official Benny Minton said.
The boat, called the Erik, was found around 87 miles south of San Felipe, a popular holiday and fishing resort located south of Mexicali.
According to an Internet advertisement, the Erik has been operating in the Sea of Cortez since 1989 and can sleep up to 42 guests.
"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.