Sounds like a bad battery. I had a similar problem last year. My battery was one year old and i only fish weekends mostly providing good weather so there really was not a lot of use on the battery. Started my boat at the dock and all seemed fine, shut off the main engine and switched over to my kicker. When it was time to head in turned the key to start the main engine and just a bunch of clicks. Lucky for me the kicker motor got me back to the dock. I looked all over my boat for some kind of short and could not find a thing, put the battery back on charge and it charged right back up. I headed out on the lake and the same exact thing happened again. I could not believe that my battery could be bad after just one year use but i decided to head down to the local auto parts store to have it tested and sure enough the battery had a bad cell. New battery and problem fixed, of course now i always carry a spare battery just in case. Only takes a couple of minutes to pull a battery and have it tested, i would start there before going crazy looking for something that may not be broken.