The sailor died after being pulled alive from the water, said Navy spokesman Cmdr. Jack Hanzlik.
"It's a sad time whenever we lose shipmates like this, so our hearts and prayers go out to the families," Hanzlik said.
The helicopter, based at Naval Air Station North Island in Coronado, was on a training mission when it crashed about 50 miles (80.4 kilometers) off the California coast. The MH-60S, commonly known as a Seahawk, was on a mission off the USS Bonhomme Richard, directly west of the Camp Pendleton Marine base.
Hanzlik said the helicopter crew put out a mayday call before the crash. Navy sailors and Marines who were training with them aboard ship arrived at the crash site in inflatable boats within minutes.
The sailor pulled alive from the water died aboard the Bonhomme Richard while receiving medical attention, the Navy said.
His name was being withheld pending family notification. The names and rank of the others aboard also had not been released.
It was not clear whether mechanical malfunction or pilot error might have contributed to the crash, the Navy said. An investigation was under way.
Hanzlik said he did not know what type of maneuver the helicopter was performing when it crashed.
The USS Bonhomme Richard is an amphibious assault craft that took Marines to Indonesia after the 2004 tsunami.
The MH-60 Sierra is a twin-turbine craft based on the UH-60L Black Hawk and the Navy's SH-60B Seahawk, according to the manufacturer, United Technologies Corp.'s Sikorsky Aircraft. It is designed to operate off aircraft carriers, cruisers, destroyers and frigates, ranging up to 100 nautical miles from the ship.