konocti harbor rents all kinds of boats, including jet ski's. They also do lake tours on the Konocti Queen. The queen is cool for people who dont have boats and have never seen the lake.

I believe the place paul is talking about is on 3rd st near the ramp.

Plenty to choose from all around the lake. These are the 2 closest. I also remember a place in Kelseyville that use to tow the jet ski's to state park, but I cant remember anything about them.