Does it not restart after you shut it off and then try to restart or does it cut off while running? If it won't restart after shutting off and it's hot then I would say vapor lock. I had same problem with my 454's and narrowed it down to the fuel line feeding the generator. Does this boat have a generator? Also, if it happens again try removing the gas caps and let the vapors escape and then try to restart. This seem to work for me until I installed a fuel cut-off valve on the generator fuel line or made sure that the fuel line was full of gas. It seems that when the generator fuel line was not pressurized vapor would flow back to the tank when hot and cause engines to not re-start. ( This assumes you have one tank for both engines and generator)