When I was working on my Alum Boat last fall, I came across some interesting things the previous owner did to the electrical   and I am not an expert by any means:
 
Here's the Don't do it........
 
Do not attach the negative side of the battery to the frame of the boat or it will ground the boat.
You do not want to do that - ever. If you plan on installing a Panel of switches (bilge,horn,livewell etc etc) install a Bus Bar.
 
It's also smart to install a Circuit Breaker as close to the Battery as possible - like real close - within a foot.
 
If you ground the boat and your hot wire manages to get cut or scuffed somehow, it will def. cause problems. It can cause sparks, fire or in the worst case a battery explodes.
If you don't ground the boat and you cut or scuff the the hot wire you will only lose the power to that accessory.