There is a similar discussion going on here: http://www.tmcowners.com/teamtalk/showthread.php?t=6569
I'm getting ready to install dual batteries in my Prostar 190 and I'm planning on using a different isolator/combiner. Specifically this unit: http://www.yandina.com/c100Info.htm
I believe there is one drawback to using a "dumb" relay. When the key is switched to ON the batteries are immediately placed in parallel. If one of the batteries is significantly drained it will pull the other battery down and equalize at a voltage level somewhere in the middle. This could all happen between the key hitting ON and START. Potentially the voltage could drop low enough to prevent the boat from starting. I have never had this happen but I think it is theoretically possible.
The Yandina combiner only places the batteries in parallel when one of the batteries reaches 13 volts, ie it is being charged. This only happens when the alternator is turning, ie the boat is running. So starting the boat will drop the voltage of the starting battery down, once the alternator has it fully recharged, the batteries are placed in parallel to allow the aux battery to charge.
I'm not trying to knock the method above, obviously it works very well. Just throwing out another alternative. I'll draw up a diagram for the setup I'm planning and post it.