A New Hampshire man drowned on Saturday in strong surf at Quansoo Beach in Chilmark, despite a dramatic rescue attempt by police, fire and ambulance workers at the remote location near the man-made cut at the barrier beach that separates Tisbury Great Pond from the ocean. Lawrence Shedd, 49, of Danbury, N.H., was pronounced dead at the Martha’s Vineyard Hospital at 3:38 p.m.

Circumstances surrounding the incident are still under investigation by police.

An eyewitness called 911 at about 2 p.m. to report a swimmer in distress. There was some initial confusion about the location and rescue workers went first to the West Tisbury side of the pond opening. When it was discovered Mr. Shedd was on the other side, a rescue team from Chilmark quickly responded, using all-terrain and four-wheel-drive vehicles to reach the scene.

Meanwhile, rescue workers who had arrived on the West Tisbury side waded across the channel, which is approximately five feet deep and 50 feet wide, to reach the victim.

The ocean was especially rough on Saturday, with shoulder-high waves breaking close to shore and larger waves breaking farther out. The area where Mr. Shedd was swimming, just west of the opening to the Tisbury Great Pond, drops off sharply not far from the shore.

The events leading up to the drowning are unclear. A press release from the Chilmark police department said rescue workers were dispatched to the Quansoo Beach opening at the Great Pond for a report of a male party who had gone under the water and was pulled to the beach.

Andrew Moore was surfing with friends on the West Tisbury side of the channel Saturday when he spotted several people giving CPR to someone on the other side. He waded across the opening to see if he could help, and watched as two people continued to give Mr. Shedd CPR. A young woman, believed to be a lifeguard, soon arrived and assisted with the rescue efforts, he said.

Mr. Moore said a young man from the Trustees of Reservations then showed up on an all-terrain vehicle on the West Tisbury side of the opening, and waded across the water holding a defibrillator in the air. An ambulance then arrived on the wrong side of the channel, followed by police, fire and ambulance personnel driving four-wheel-drive vehicles and all-terrain vehicles, the veteran Vineyard surfer said.

Mr. Moore lauded the workers for their rescue efforts.

“Everyone got there as quick as they could; everyone was doing their job. It was a pretty amazing effort,” he said.

The barrier beach at Quansoo near the pond opening is remote and lies a good distance from the parking area, which is itself about a 10-minute drive from South Road.

Chilmark police Chief Brian Cioffi was off duty and at his home, not far from the Quansoo Road on Saturday, when he received word about the swimmer in distress. He grabbed a defibrillator and a backboard from his cruiser, jumped on his own personal all-terrain vehicle and headed to the scene.

He was among the first to arrive and was soon joined by members of the Tri-Town ambulance, several Chilmark police officers and a four-wheel-drive rescue truck from Oak Bluffs. Rescue personnel from several other towns also arrived, the chief said.

He said rescuers tried to resuscitate Mr. Shedd using CPR and a defibrillator for a long period of time. The resuscitation efforts continued even after Mr. Shedd was loaded onto the back of a four-wheel-drive vehicle and taken off the beach. He was then transferred to an ambulance and transported to the hospital, where he was pronounced dead.

Chief Cioffi praised the efforts of the police, fire and ambulance personnel.

“The response was incredible,” he said.

West Tisbury police Sgt. Jeffrey (Skipper) Manter was among those who initially responded to the wrong side of the channel, and was one of about a half dozen rescuers who waded across the channel. When he made it to the other side, he said CPR was already being performed by two bystanders and the female lifeguard, although other rescuers took turns after fatigue set in.

“It was a joint effort, everybody did their job and helped out,” Sergeant Manter said.