If you really want to paddleboat....
There are so many more beautiful places in San Francisco, unless you really want to paddle boat, I'd give this place a miss.
You can walk the perimiter of this lake in about an hour. Some interesting things along the way include a stone bridge, a man-made waterfall, Chinese pagoda, and **lots** of turtles sunning themselves on the banks. You can also hike up the waterfall to to get a nice view of the city (on a clear day that is!)
The boat house and snack bar are a bit shabby. There are lots of picnic tables so you can bring your own lunch to enjoy. And the rental fee for the paddle boats (which seat up to 4) was $17/hour. They also rent bikes.