An autopsy Monday showed no obvious reason for the death of San Francisco resident Robert T. Stewart, 38, who died Sunday, soon after he got into the ocean to dive for abalone.

Stewart was the seventh person to die this year while abalone diving on the North Coast.

Stewart didn't drown because he began having trouble before he'd put his head underwater, said Sgt. Mitch Mana of the Sonoma County coroner's office.

A pathologist now will look for the cause of Stewart's death from toxicology tests, a review of medical history and possibly microscopic tests to look for any sign of disease, Mana said.

Stewart and two friends went into the ocean off Shell Beach at the Sea Ranch about 12:30 p.m. Sunday. It was a clear, sunny day, and the water wasn't rough.

"His buddies told me . . . within about a minute, their friend complained he was really tired," said Sgt. Eric Thomson, who responded to the call for help with the Sonoma County sheriff's helicopter.

"So they escorted him to this rock about 100 yards offshore," Thomson said.

They put his hand on the rock, telling him they would rest. "He just went unconscious," Thomson was told.

The friends yelled for help and pulled the unconscious man up onto the rock. A fisherman on shore heard them and called 911.

Because the man was so far offshore, the helicopter crew was called to help.

Thomson said they arrived in about 15 minutes. They found the two friends conducting CPR on the man on the rock, being hit by waves.

The helicopter crew dangled a 100-foot line to the rock, plucked the man up and carried him to paramedics waiting at the end of a nearby Sea Ranch cul-de-sac.

Stewart was declared dead at 1:13 p.m.

For past coverage of the slayings, go here