The 18-year-old man was reported missing at 6:09 p.m. Friday by other windsurfers in the area who did not see him when they came in at the end of the day, Coast Guard Lt. Simone Mausz said.
The Coast Guard sent a helicopter and three vessels to the area, while San Mateo and Foster City emergency crews also responded, Mausz said.
The windsurfer was eventually found clinging to his surfboard at about 1:30 a.m. near the bridge, Mausz said.
She said he had mild signs of hypothermia but "had on a half wetsuit that protected him a little bit from the temperature of the water."
Some of the other windsurfers said the man may have been inexperienced, leading to him becoming stranded in the water, Mausz said.
"They said it looked like he was struggling, but we really don't know," she said. "It was pretty windy yesterday so that also could be it."