The above method is how I have my boat wired and has served me trouble free for many years. It works well and is completely automatic but if you require more control over your batteries you can add a DPDT switch in your dash to manually control the relay. I used a DPDT switch in conjunction with the relay in my Jeep so I could manually override the ignition circuit in certain circumstances. I have never had the need for more control in my boat but I outlined the details below for those who are interested.
All the wiring is the exact same as described above except at a DPDT switch is added to the ignition circuit. A schematic has been drawn below detailing the circuit. The relay is connected to terminal A of the switch and a jumper wire is then connected from terminal A to terminal F. The ignition circuit from the key is then attached to terminal B. A new wire is sourced from a constant positive source (back of the key also contains a constant positive) and connected to terminal E on the switch.
Now you have three options depending upon where the DPDT switch is located. With the switch in the middle position all circuits are open the batteries are always isolated regardless of key position. With the switch down the system is identical to the automatic mode in the original installation. The key automatically controls the isolation. With the switch in the up position the batteries are connected 100% of the time regardless of key position. The addition of the switch does provide an extra level of control since you can select how you want the batteries to be isolated manually. Plus it still gives you the option to put the system in an automatic mode. I have found when installing the switch in boats they stay in the automatic position 99% of the time anyway so I usually do not bother with one.