3rd batt install (fuse question)

I'm installing a 3rd batt. My boat came stock with 2 batteries, they're already seperated. 1 is for starting, the other is for the stereo. I want to add another for the stereo. The way my boat is set up, the 2 that are in there already, are on the pass side all the way in the rear. The new one would be installed on the drivers side in the rear. I think there would be about 8' of wire in between.

My question is, do I need to put a fuse in between? If so, do I need a fuse on both positives? I know it's always best to put a fuse on everything. But there doesn't seem to be a fuse on the wire going from the alt/starter to the battery now. So do I really need one? If so, how may amps?