Fernandez was with two friends in a 32-foot motorboat that smashed into a jetty that delineates the northern section of Miami’s deepwater channel, said Officer Lorenzo Veloz, a spokesman for the Florida Fish and Wildlife Conservation Commission. The channel, called Government Cut, is used by cruise ships and freighters to head into and out of the Atlantic Ocean. It sits on the southern tip of South Beach, a popular tourist destination in Miami Beach.

Officer Veloz said it could not be determined whether alcohol or drugs had been involved in the crash because the boat was badly damaged. Toxicology tests will be performed, he said. None of the men were wearing life vests.

Fernandez overcame great odds to reach such professional heights. He was jailed as a teenager for attempting to defect from Cuba. He succeeded on his fourth try, at age 15, saving his mother from drowning along the way.

He emerged as a baseball star at Braulio Alonso High School in Tampa, Fla., and was chosen by the Marlins in the 2011 draft. He reached the majors in 2013, when he was named the National League rookie of the year. In each of his four major league seasons, Fernandez had an E.R.A. below 3.00.