of course this is the one time my tool bags and compression gauge is not in the back of my truck.

compression check would be the 1st thing i would do if i had my tools

next step tomorrow thanks guys

so i will roll it over and wd40 the rear cylinders until

need to taste the water coming out of the cylinders time to crawl back down in the bilge

Don't forget it might give you a false high #reading because its a wet cyl. confirm with leak down.look close at the riser, spray brake clean on them, it helps bring up any cracking

Doesn't that have a closed cooling system? If so the only way to get water in is from the exhaust.

The 496 has closed cooling. If it was a head gasket or intake leak it would be antifreeze coming out of the cylinders and you would be low in the reservoir.

I had a friend that chased this same problem in his 2003 496 formula 330 sun sport. He changed manifolds , risers and starters , it would only do it once in a while. And then he finally listened to me and pulled the cylinder heads and and found the exhaust valves were not sealing and were causing water to be sucked back into the cylinder .

496's are dryjoint, you can pressure test manifolds without even pulling them........

Tommy , Those 496's are freshwater cooled , if it was a head gasket , coolant tank would be empty or it would have burst a hose. Are the manifolds new ? If they are the fact aluminum , they rot and dump water internally.
I know you said they have 500hrs on them , but the height of the standard elbow is right on the verge of being to low on some installations. Put enough fat asses in the back corner of cockpit and you could have trouble, especially if the motor is already having an issue. We put 6" taller elbows on our parasail boats with 496's after seeing our competition kill two motors this way. Good Luck!!!

Sounds like a leaking exhaust manifold to me.

You need to disable spark and fuel - Leaving spark plugs wires hanging is never a good idea