An ACR (Automatic Charging Relay) is often part of such a system as well.
Make sure the wire is marine grade, and not just some stuff from Home Depot.
If you don't already have a battery switch, you should install the new one outside of the engine bay (if possible, not always practical on smaller boats) so that you can disconnect both batteries electrically in the event of a fire.
This page/diagram should help:
http://www.westmarine.com/webapp/wcs...nersRelays.htm