So let me get this straight the boat dies, but then will not restart for a minute or so? If you crack the throttle open in neutral will it start right back up? That is testing to see if the IAC is cutting the motor or a possible lean run condition. Another question is going to be how does it die? Like someone shutting the key off or like a slow rich stall? Does it ever stall off idle?
There are many variables that can cause this scenerio and things rarely get better but always tend to get worse, and will haunt you at the worst possible time. If there is a vacuum leak it can cause it to stall. The absolute best posssible way to check for leaks is with a smoke machine that most high end auto shops will have. If they don't they are cheap! You can try cleaning the throttle body out, but try to use a non caustic cleaner and keep it out of the electrical components.
Scanning the computer might tell you something but the chances are there are no codes. The data stream can often be useful to see what caused it to die. Same with scoping it to see if it is an ignition cut caused by a bad sensor.
You could always get the idle set up a little and disconnect the IAC and leave it there and see if it still dies. That will set a code for sure
Not sure if the factory scanner for boats can up the idle, like you can on many cars.