The rough weather sent water surging over the dock about 10 a.m. and flipped over the gangway used to load and unload passengers. The mishap damaged hydraulic and power lines and also flooded a souvenir stand and rest rooms.

About 280 visitors were brought from the cell house to an auditorium near the dock and shown films, including the 1996 thriller about Alcatraz, "The Rock," to pass the time.

After the seas calmed, a smaller ferry able to use the island's utility dock was dispatched to carry the stranded passengers and employees in two trips back to Pier 41. The second trip did not return to Pier 41 until about 4 p.m.

The Blue and Gold Fleet, which operates the ferry service, was forced to cancel nine scheduled trips to the island and to issue refunds to ticket holders.

It was the third time that people have been confined on Alcatraz against their will since the federal prison closed in 1963, Weideman said. Twice in recent years, rough weather has stranded visitors on the island for as long as six hours.

One of them was Leon "Whitey" Thompson, a former inmate of the prison during the 1950s and '60s who had served a sentence for armed robbery.

He had returned to the island yesterday to autograph copies of his autobiography for visitors in the island bookstore.