The old saying "If it ain't broke, don't fix it" comes to mind here. If the current battery is working fine as far as starting the engine and isn't causing you any other placement problems (like you want the space for something else) then I would leave it where it is.
That said, if I was starting from scratch I would try to place the batteries as close to the engine as I could get them. The starter draws more current than anything else it is just better overall to keep the starter wiring short. If you have a center engine boat the bow might be as close a spot as any. For a Vee drive, putting the batteries in the back is definately the way to go.
I would recommend keeping the second battery relatively close to the first battery. The only real issue is that you need to connect the two batteries together with large battery cables which could get expensive if the batteries are far apart.
Having the batteries forward would certainly benefit a stereo installation, which I assume is the reason you want to install a second battery.