A powerboat navigating a channel in an area dotted with boggy salt marshes ran aground south of Seaford, killing three people and seriously injuring four others, authorities said.

The 40-foot Pleasurecraft speedboat plowed onto marshland in Bellmore Channel, east of Gooseneck Bridge on Wantagh State parkway, on a moonlit Sunday night off the island’s southern shore, authorities said. The boat’s captain, George Canni, 65, of Copaigue, was thrown off the craft and pronounced dead by rescuers when they arrived.

Capt. Joe Frohnhoefer, the founder of SeaTow, a marine assistance organization with headquarters in Southold, said one of its boats was the first to arrive at the crash site, near a bridge on a parkway that leads south to Jones Beach.

The boat was about 150 feet from the water, Frohnhoefer said.

The Nassau County Police Department said officers responded to the accident at about 7 p.m., along with the U.S. Coast Guard, Wantagh Fire Department and the New York City Police Department. At least two people were airlifted to nearby hospitals, according to the Coast Guard. The survivors remained hospitalized Monday morning.

Nassau County police were investigating the cause of the accident. The other two victims who were killed in the crash were identified as the captain’s wife, Theresa Maniaci-Canni, 46, and Joseph Sugamele, 50, of Massapequa. Police did no release the names of the four who were injured.

Frohnhoefer said he has seen four similar accidents, usually occurring when boats run up on sandbars while trying to navigate the complex network of channels and bays.

“It’s a lack of knowledge, a lack of boating knowledge,” he said.

Frohnhoefer said it was too early to speculate on a cause of the crash.

Copyright 2009 The Associated Press