Battery ground problem? advice appreciated

I recently had some issues with my batteries and had to take them out to charge them up. I have an 18 foot Aquasport center console with 1 marine starting battery and 1 regular battery for running the electronics (just a fish finder and radio basically). I went install my batteries after charging and when I went to connect the positive terminal on the starting battery, the ground terminal was already connected, and felt a current going through my hand. The battery switch was off and no electronics were on.

I am not sure what exactly this means and what damage it could do. I assume it is a faulty ground but I can't find where it is. I am only going to have my boat in the water for another 3 weeks and I am wondering if I can't find the problem if I could still use my boat safely. Will this drain my battery? Will it cause any other problems? Is there a way to handle rig it for 3 weeks till I can get it to my mechanic?

Thanks for any help.