No one was injured in the attack Tuesday, which took place 25 miles east of Gulf Shores, where two swimmers were attacked by a bull shark Friday, Coast Guard spokesman Charles Clark said.
The Pensacola, Fla.-area boaters, whom the Coast Guard did not identify, saw a large shadow in the water at about 2:30 p.m. as they were sailing west of Pensacola Beach, Clark said in Wednesday's editions of the Mobile Register.
The boaters said that as they moved closer, the shadow looked like an 8-foot bull shark, he said. The shark apparently lunged at the rear of the hull, ripping its rear swim platform.
Two men were injured in Friday's attack - Alabama's first confirmed shark attack in 25 years - prompting officials to shut down a 30-mile stretch of land at the height of tourist season until lunchtime Saturday.
Chuck Anderson, 44, lost his right arm above the elbow. Richard Watley, 55, was bitten on his right hip and right arm.
Gulf Islands National Seashore Ranger John Bandurski said the beach would remain open despite Tuesday's incident.