This is So Cal man, you can boat year round. Currently the lake is warmer than 80 on the surface and it will only drop to around 50 in the winter LOL.
Seriously, you can boat until Nov. easy. If you use the city (eastern) ramp, watch out for the poor condition of the docks themselves. If you use the marina ramp, do not put your front wheels anywhere near the water. Probably not much police presence after Labor Day but you will always run a 50/50 chance of dealing with the ramp Nazi at the Marina West ramp. Enjoy!
Oh and the bathrooms in the campground at Marina West are way cleaner then anywhere else on the lake.