Need help with neighbor's boat overheating
 

OK my neighbor has a boat with a mercruiser 350 and Alfa one. Seems like idling in the water for any period of time, like a long no wake zone causes the motor to heat up. Once it hits about 200 degrees, it sputters a bit and then dies.

We have changed the out drive water pump, engine water pump, butter flies in the exhaust going out the back, plugs, wires thermostat. Anyone have any ideas? It seems to be OK running wide open. Maybe the ignition module getting hot and breaking down? Maybe the out drive water pump bad on install? I am at a loss. This is indeed only a low speed idle issue. Could it be the exhaust manifolds? He pulled the risers and they were OK but didn't take off the manifolds. There has to be a reason that it heats up and dies at idle going through the no wake zones.

Help

TIA
Rob