At about 5 p.m., the sheriff's marine unit responded to a cove at the lake, sheriff's Sgt. Ed Hoener said. The Geyserville Fire Department and park rangers from the U.S. Army Corp of Engineers also responded.
Rescuers arrived to find someone performing CPR on the victim, who was then loaded onto a sheriff's boat and taken to the Lake Sonoma Marina where waiting paramedics took her to a local hospital, Hoener said.
The woman's name has not been released because her family has not yet been notified of the incident, according to Hoener.
Alcohol appears to have been a factor in the incident, he said.
The woman, her boyfriend and their dog were in a boat anchored in the cove, and were drinking and swimming when she dropped below the water's surface and did not come back up, Hoener said.
Someone in a nearby boat called 911, but poor cell phone reception made it difficult for the caller to give the exact location.
The reporting party then rushed to the marina and found someone who knew CPR. That person came back out on the boat with him to try to resuscitate the victim until rescuers arrived.
As of this morning, the woman remained in the hospital's intensive care unit in critical condition.