I live in N. Ca. also and all I have done to boats that I store outside is use a canvas cover and then a blue plastic tarp (supported to let rain off). I also put a drop light in the boat with a 60w bulb running 24-7. If it's going to be in the 20's (really rare here) I throw another drop light in the engine compartment. I also check the bulb at least once a week. Usually just verifiying that there is no frost on certain areas of the blue tarp (such as the high spots where the heat goes). All the bulb has to do is keep the insides of your boat a few degrees warmer than the outside and and mold won't grow.........at least it has not in MY boats that I store that way.