Try these guys. I'm not sure if it's indoors or not. Secured Self Storage 49 Cosmic Ct Copperopolis, CA 95228-9300 3.72 miles from the center of Copperopolis, CA (209) 785-9400
there is a Place right on the water called Drifters Marina. They have coverd self storage garages. Don't know for how much. They aslo have slips 4 rent. I don't have a phone # but do a search I'm sure you can get the # if you can't find it let me know Ill put U in contact with the owner