A scuba diver who died Saturday while exploring a sunken battleship off Mission Beach was found in the ship's boiler room, a compartment that had been sealed off to divers.

An autopsy completed yesterday on the body of scuba instructor Steven O. Donathan, 50, of Point Loma concluded that he drowned while in the Yukon, a 366-foot-long warship scuttled five years ago in 100 feet of water to create a world-class diving attraction.

Divers who recovered Donathan's body Tuesday found him pinned against a wall and entangled inside the boiler room on the sixth deck in the bottom of the ship, said lifeguard Lt. Nick Lerma.

Although the entrance to the boiler room had been welded shut, someone apparently had pried it open.

"It was a hazardous place," Lerma said. "Why he was there, we are not sure."

Donathan's air tanks were empty and there was no safety line in place to lead him out of the ship, Lerma said. In addition, Donathan entered the bowels of the ship with his diving student but became separated from him, Lerma said.

A cardinal rule of scuba diving is to immediately surface if you can't quickly find your diving partner.

"There are risks associated with diving, and when you push the limits for whatever reason, the consequences tend to be more severe," Lerma said.

Many divers have an "explorer attitude" that motivates them to "see what's around the corner," said Bill Reals, a friend of Donathan's.

Reals said he can't imagine that Donathan removed the barrier blocking entry to the boiler room.

"We're all concerned. We want to understand what happened," Reals said. "I think we're all scratching our heads."

Donathan is the first diver to die inside the Yukon, a decommissioned Canadian warship purchased by the nonprofit Oceans Foundation and brought to San Diego to entertain divers and become an artificial reef.

After the ship was scuttled 1.85 miles offshore from Mission Beach, Navy divers cut three rectangular access panels from the vessel's three-eighths-inch thick steel hull to make it safer for recreational divers.

Besides working as a diving instructor, Donathan was among the area's elite technical divers, who are capable of diving in deep water using a special mix of gases.

His friends said he had completed 40 to 50 dives on the Yukon.

"He knew that vessel inside and out," said Lerma, the lifeguard. "He was a very experienced diver and he was familiar with that wreck."

The Divers Alert Network at Duke University, which keeps track of diving accidents in the United States and Canada, reported 89 diving fatalities throughout North America in 2002, the most recent year for which statistics are available.

The leading cause of death, accounting for 47 percent of the cases, was drowning. The next leading cause, 24 percent, was divers running out of air.

Equipment failures are extremely rare, and 1 percent or less of the victims die from decompression sickness.

"Nearly every diving fatality is preventable," said Laurie Gowen, a dive medic with the network.

Lerma said a special underwater investigative team consisting of lifeguards and police divers will examine the events leading to Donathan's death and issue a report.

• San Diego Union-Tribune