250 optimax smokes very intermitently

[edgeflyer]

My opti smokes out of the exhaust arround mid throttle. It a white smoke that dosn't linger. It seems to disipate at an almost evaporation rate. the boat runs great and has just been checked out by the dealer. It smokes inrtermitently. My water presure at high throttle is only 7 psi , but temps never get above 132 degrese. It did have an overheat before I bought it that I found out about, but like I said all cyl's check O.K. and it runs perfect. Could it be possible that the head gasket is leaking water into a cyl , and causing a water vapor smoke, or could water be backing into the muffler , then evaporating out in the form of smoke? any info on why my water presure would be low?