A 51-year-old vacationer faces possible paralysis after being thrashed against the South Beach shoreline by fierce waves Tuesday afternoon.

George Medford of Ballston, N.Y., who had been bodysurfing close to the South Beach shoreline with his son, snapped several vertebrae in his neck when he struck his chin against the sand, officials said.

A lifeguard rushed to the scene after noticing Mr. Medford being rolled over and over in the breaking waves. The lifeguard pulled the swimmer from the water and held his neck still until emergency medical technicians arrived.

When EMTs and police arrived shortly before 4 p.m., Mr. Medford had a pulse but was unable to move or breathe, officials said. Paramedics revived Mr. Medford before loading him onto a board and transporting him to the ambulance with the all-terrain vehicle.

Mr. Medford was airlifted early Tuesday evening from Martha's Vineyard Hospital to Boston Medical Center, where the doctors evaluated severe neurological problems.

Boston Medical Center listed Mr. Medford in critical condition yesterday afternoon.

Emergency medical teams responded to two other calls at South Beach Wednesday. A 12-year-old boy sprained his left knee after skimboarding with friends shortly after noon. He received treatment at Martha's Vineyard Hospital and was released the same day.

Another juvenile complained of spinal pain yesterday afternoon at South Beach. Edgartown EMTs transported him to the hospital shortly after 5 p.m. Wednesday. Doctors at Martha's Vineyard hospital treated and released the patient the same day.

An onshore breeze exacerbated the strong shorebreak at South Beach this week.

"We never know what Mother Nature will hand us," Edgartown park department administrator Pamela Dolby said yesterday, commending Edgartown lifeguards for their response to emergencies this week.

Ms. Dolby advises beach-goers to use extreme caution when swimming.

"Pay attention to the conditions of South Beach. It's not necessarily a safe beach for young children," she said.

"It's a beautiful but dangerous beach," Ms. Dolby concluded.