Hi Mel,
I was gonna e-mail ya and saw this.
That sounds like a back-fire from air getting sucked into the exhaust from reversion. Did it happen when decelerating, letting off the gas? If your motor has a performance cam with high overlap it can do that. There is a very brief moment when both the exhaust & intake valve are open and a dose of unspent CO (carbon monoxide) or in your case oxygen that is leaking in will go into the combustion chamber and cause a lean condition or pre-ignition.
Are you guys gonna take your boat out again this year?