Unless the boat was swamped then bad cooler.
Don't take your boat to that mechanic again.
He should have been able to diagnose the problem and not just take your reccomendation to flush it and call it good.
'06 X2 MCX
"I understand why some people may not want to do this the way I have recommended but I can't understand the death grip some people have on a toilet plunger with a hose fitting." -JimN