Francisco Fernandez, 66, was flown by the REACH air ambulance to Santa Rosa Memorial Hospital after the crash, which happened along the lake's Warm Springs arm on the southern end of the lake at about 3:40 p.m., according to the Sonoma County Sheriff's Office.
Speed and inexperience operating the personal watercraft appear to have contributed to the crash, said sheriff's Sgt. Ed Hoener, head of the boating unit. Alcohol is not believed to have been a factor.
The accident happened near Madrone Point, a campsite accessible by foot or boat only, according to the sheriff's office.
Sheriff's helicopter Henry-1 found Fernandez being attended by other boaters who had seen the crash and stopped to help, Sgt. Dave Thompson said.
The jet ski was on the rocky shore and Fernandez was halfway in the water, Thompson said.
The helicopter crew used a long line to get to Fernandez and flew him to a nearby landing zone, Thompson said.
Hoener said it's fortunate that Fernandez did not suffer more serious injuries. It was the first boating accident reported at the lake this summer, Hoener said.