After market battery isolator question

[fishingwithblue]

I was planning on installing the Blue Seas system Whatknot did on his boat. It has a battery switch with on, off and combine for emergency start and a relay to isolate the batteries. Hook it up and turn the switch to on and it only uses the cranking battery when cranking. Then when I turn the boat off and have the vhf, GPS, radio and bait tank running it only draws from the 2nd batt. When you run the boat it charges both. Like Whaknot said set it and forget it. But now, after reading through the book i'm worried I will screw up the engine installing this system. 200 hp Evenrude Direct Injection 2004