Mercury 40 hp Classic, water in bottom cylinder

Hi, I have a Mercury 40 hp Classic Oil Injected outboard serial no: OD229057 Brunswick Corp. Fon Du LAC, Wis. USA. I am not sure of the year. The trouble I am having is a small amount of water is entering the bottom cylinder after a shot time of running the motor causing the spark plug to stop firing on the bottom cylinder. The motor has never been in salt water so I ruled out corrosion through the cylinder wall. The engine looks like new, the paint is not discolored so I ruled out over heating. It came off a pontoon boat. What else could be the problem? Please help save my families camping trip this summer by giving me a hint of what to look for God Bless! Gary DiCenso

I would do a compression test and go from there.

Thanks for the responce! Did a compression test and all cylinders were 125 psi.