retsey.
I rewired my boat last spring...
I ran 4awg from battery (in stern) to cuddy where my fuse block is located. The fuse block has my am/fm radio, subtroll, hummmin bird fishfinder(1158C), Lowrance (5200) GPS and there is no interference. I do not have a switch to turn it off, however, if I don't use the boat for an extended period of time, I simply pull the fuse.
I went with the following...
http://bluesea.com/category/81/21/products/5026
Ray