1996 challenger losing power, water in cylinder

[BCSwider]

I bought a 96 challenger that hadn't run for a year. The rotary valve was seized but the engine looked good so I had it broken down, cleaned, and put back together. The boat ran fine for an hour then the o ring on the cylinder head split dumping water into the rear chamber. A new o ring fixed it for another hour untill that one went, and there was water in the chamber again. I replaced it and it ran fine for 1 minute until at full throttle i was only getting 3000 rpms. Pulled the plug, no water. Cleaned and replaced it, 1 minute of running perfect then the same thing happened. The boat is loosing the rear cylinder after a minute of running fine, no matter how many plugs I change. No one knows what is happening and my boat is sitting in the driveway. I cleaned the fuel filter, the carbs were cleaned a week ago, there is spark in both cylinders, no more water is getting in through the head, and I have 180 psi in both cylinders with an r&d head. I need serious help or this thing is going to drive me crazy.