Check your spark plugs for signs of water. It's the easiest thing to check first.
Usually the riser or manifold is the first failure item. Followed closely by a head gasket issue .
It would be best to do some checking and preventive cleaning and lubrication before letting the engine sit for an extended period of time.
Again, I did not think much of it because I heard some one at the marine talking about how they had water in the fuel and we fill up at the same place. I figured I would put some dry gas in and problem solved.
Ayuh,... There's No way water in the Fuel could ever cause Water on the sparkplugs...
The Manifold has to be disassembled to Check it,...
The Block can be pressure tested by draining,+ pressurizing with Air...