Three scuba divers hired to probe the sunken collier Marine Electric for evidence in a $100 million negligence suit say they were run off at gunpoint last Saturday from the waters off Chincoteague, Va., where the ship sank in February with a loss of 31 lives.

The divers, employed by lawyers for the deceased crewmen and their families, told the Coast Guard they found a vessel resembling a clam boat at the wreck site 30 miles offshore when they arrived in their own 24-foot craft at the spot where the 605-foot coal ship went down.

Crewmen aboard the clam boat, which the divers estimated was between 60 and 80 feet long, warned them to leave the area, according to a Coast Guard report. One scuba diver told the Associated Press that the mystery crewmen unfurled a flag displaying a skull and crossbones--the traditional flag of pirates--to back up their threat.

The scuba diver, Jeremiah Shastid of Waldorf, Md., said that he and his fellow divers ignored those warnings, descending 120 feet to the ship.

However, when they came up about 50 minutes later, Shastid said, two crewmen on the other boat "came out with what appeared to be M16" automatic rifles.

The Coast Guard report said the divers then attempted to radio for help. When they were unsuccessful, they left the area, the report said.

After receiving the report by telephone at 9:30 a.m. Sunday, the Coast Guard sent a helicopter to the site from Elizabeth City, N.C., and diverted the cutter Port Huron to patrol the area.

A Coast Guard spokesman declined to say yesterday whether a vessel resembling the "clam boat" had been sighted.

"The incident is under investigation," a public affairs spokesman said. "No further information will be released at this time."

Who the gunmen were or why anyone should be guarding the wreck of the Marine Electric remained unclear yesterday.

Shastid said he and the other divers had been employed by "five or six law firms" to help determine why the 39-year-old ship capsized in a gale Feb. 12 and sank in about 120 feet of water.

He said he and the other divers had made eight previous dives on the ship.

Attorney C. Arthur Rutter of Norfolk, who represents the families of some of the dead, confirmed that another Norfolk attorney, Henry Howell, a former Virginia lieutenant governor, had hired the divers "to find out everything they can about the wreck" and return with pictures of its condition.

Howell remains in a hospital after recent surgery and was unavailable for comment.

Rutter said that more than $100 million in damage claims have been filed against the owner of the vessel, Marine Transport Lines Inc., of New York, including those claims filed by lawyers for the owner of the cargo.

"But I don't know why anybody would be out there with machine guns," Rutter said.

Marine Transport vice president Henry Downing said that he knew nothing of either the divers or the gunmen.

Marine Transport's insurance company, London Mutual Corp., "would do nothing like that without telling us," he said. "We talk to each other all the time." Downing said Marine Transport had contracted with Steadfast Marine of Falls Church to make a sonar survey of the broken ship.

The survey was completed Friday without incident, he said, and will be followed up as soon as weather permits with remote underwater photography of the wreck.

Downing said he had received calls from about 30 diving clubs along the East Coast seeking permission to dive on the wreck. "We can't really give anybody permission," he said, "but then we can't really keep somebody out there to keep people off, either."

The wreck has already been inspected by Coast Guard divers, and reportedly is lying in two pieces about 600 feet apart.

Shastid said divers on the clam boat were working the other end of the wreck and may have been looting it.

"There are a lot of things of value down there," he said, including the ship's safe.