Frustrating Issue Please Help

I hope this won't be to long winded.
I have a 1988-89 Bayliner Trophy with a GM 5.0 4v engine. When the engine runs normally it purrs like a kitten. However, on two separate occasion this engine has left me worried I will not get home. Here is what occurs.

After having run the boat around Erie for an hour or so at full power with no issues I shut it down. Do some perch fishing or trolling for several hours then go to fire it up and it does not want to start. Have to hold full throttle down. When it does start it will diesel a bit and sputter. Both times i have been able to get home with no issues.

Yesterday It did it again but when i got to the dock it dieseled and conked out. I tried to restart and it would start but run very rough and die out. It was as if it would not respond to the throttle at all. If i tried to rev it the engine did not respond and died.

Things I am concidering are

water in fuel> but it should run poorly all the time i think
Bad coil dying out when hot or heat soaked
Bad carb>but that should also run poorly all the time i think.

Please advise before i go to the marina and get finacially raped. I perfer to do my own work but this one has me stumped.

Thanks

Re: Frustrating Issue Please Help

after a run, pull all 8 spark plugs and see if water might be entering the cylinders from a failing exh manifold or riser/gasket etc.
it never hurts to throw a coil on an old boat - they are cheap
ever do a tune up? fuel filter? how old are the points? plugs?
then on to checking fuel pressure between the carb and fuel pump - should be around 7 psi