A Quincy man vacationing on Chappaquiddick was killed yesterday afternoon after being sucked into the propeller blades of a 47-foot white cigarette boat piloted by his best friend, William O'Connell. Mr. O'Connell, a prominent Quincy developer, now faces charges of operating a motorboat while under the influence of alcohol and fleeing the scene of a boating accident.

He will be arraigned in Edgartown District Court this morning.

The tragedy struck in Edgartown harbor on an otherwise festive Fourth of July, just as crowds started lining the downtown streets in anticipation of the annual parade.

But while kids clutched flags and waited for candy to be thrown from the floats, emergency rescue crews were rushing to the harbor on a priority one call to see if they could save a man with severe abdominal wounds who had just been pulled from the water by the assistant harbor master.

It all started after William B. Sanderson, 62, helped his friend's son and two grandchildren climb off Mr. O'Connell's performance speedboat called Thunder Enlightening. The speed boat had come ashore near Chappy Point Beach on the Edgartown outer harbor at about 3:45 p.m. and was pointed in, resting on the beach. But the big boat was unsteady in the current, police said.

"[Mr. Sanderson] got out to help steady the boat and slipped under and struck the propeller blades,' said state police Sgt. Robert Moore.

Assistant harbor master Michael Hathaway was very close by, manning the pump-out boat, and witnessed Mr. Sanderson as he was sucked under the boat. He responded immediately, helping to pull Mr. Sanderson from the water.

"He had severe wounds to the abdomen," said state police Sgt. Jeffrey Stone.

Eyewtinesses said it was a gruesome scene. "It was a mess," said one official at the harbor master office less than two hours after the incident.

Another man, who had helped to pull the victim into the harbor master boat, said the water was red with blood.

Mr. Hathaway rushed the victim to the dock by Edgartown Yacht Club. "An ambulance met us there in less than 10 seconds," said harbor master Charles Blair Jr.

Emergency medical crews could be heard on a police scanner, saying that Mr. Sanderson was in cardiac arrest and they were attempting cardio-pulmonary resuscitation (CPR).

"We had a pulse," an EMT said. "But we lost it."

At the Martha's Vineyard Hospital, the emergency room trauma team was unable to revive Mr. Sanderson, who was pronounced dead about a half hour after arriving at the hospital, according to Dr. Alan Hirshberg, director of emergency services.

As medical crews tried their best to rescue the victim, the speedboat captain, Mr. O'Connell, 63, had turned his boat around in the outer harbor and headed out, according to police, who dispatched a Coast Guard boat to follow him.

By the time Mr. O'Connell returned to the Oak Bluffs harbor, a welcoming party made up of state police, Oak Bluffs police and Coast Guard officials had assembled there and placed him under arrest.

He was charged with two felony counts: negligent operation of a motorboat causing death and leaving the scene of a motorboat accident where death occurred.

He also faces two other charges of operating a motorboat under the influence of alcohol and operating a motorboat within 150 feet of a swimming beach.

The beach closest to where Mr. O'Connell steered his high-speed power boat is the Chappy Point Beach, owned by the Martha's Vineyard Land Bank. The Chappaquiddick Beach Club lies a quarter-mile farther to the east.

Mr. O'Connell was taken to the Dukes County jail in Edgartown where he was booked and then released on bail. He was expected to be arraigned this morning in Edgartown District Court. Sergeant Stone would not say whether Mr. O'Connell submitted to a breathalyzer test that would have given police his blood-alcohol level.

"Police made certain observations" that led them to charge him with operating a boat under the influence of alcohol.

According to police, Mr. Sanderson was not only a friend of Mr. O'Connell, but also a business associate. It is known that Mr. Sanderson was a real estate broker who worked for a Marina Bay real estate company in Quincy.

Mr. O'Connell was one of the primary developers of the Marina Bay complex and is also one of the principal developers of a $54 million golf course project in Quincy, paid for, in part, by public funds from the Big Dig project.

Police said the two men were best friends. Information obtained by the Gazette shows that the men were avid runners, who came to the Island at least twice to run in five-kilometer road races.

Yesterday's boating tragedy was hardly the only medical emergency of the day. On a day that was searing hot, emergency medical technicians, ambulance crews and the hospital emergency room staff spent much of the Fourth of July holiday dealing with a steady stream of accidents and people who succumbed to the muggy heat.

There were at least two moped accidents, one of them at Five Corners in Tisbury. A motorcyclist was injured in yet another accident.

There was double coverage - two physicians - on duty in the emergency room, and Dr. Hirshberg was called in for back-up even before Mr. Sanderson was rushed in with the propeller wounds to the stomach.

By 8 p.m., Dr. Hirshberg's voice was tired. "The weather is very hot and sticky. We have double docs, and we have many people here being treated," he said. "But that's what happens on the July Fourth weekend."