Adding a second battery

I want to add a second battery in my boat just so I will always have a backup battery to start the boat if we decide to just hang out and listen to music or fill sacs without the boat running. My question is, does it matter how far away the second battery is? The way my boat is set up I would like to put the battery about 7'-8' from the perko and other battery. Will this work or do I need them to be right next to each other. Thanks in advance.

Put the battery where-ever you want. Just make sure it is secured and wired appropriately. Make sure you put it in a case to prevent it from being accidently shorted. If you have the room, it would make sense to keep them together.

Frank is right, it does not matter where you put the battery BUT the longer the distance from the battery to the starter the heavier the gauge of wire you need to run.