Daniel wrote in message
Ralph

Not so uncommon and may be due to a malfunctioning of the exhaust/raw
water discharge circuit.
Sometimes it is just a vent that is plugged and permits a backflow of
water into the cylinders after shutting off the engine. The water, then,
seeps between the pistons and the cylinder walls into the sump.
Sometimes this problem, if not understood early, may cause the blow up
of the engine head at the next attempt to start the engine.

I second that. Also look in an exhaust system design flaw. Could the
seawater siphon back?

Jacques
http://bateau.com