Oh for goodness sake... bad information alert!
There is absolutely no way for water to be "forced" into the cylinders by turning the water on prior to starting the engine. I would love to know the supposed path the water would take to do that. Sheesh.
It sounds like the you did not address the real cause of the water intrusion when you took it apart. How did you determine it was a bad head gasket? When you took everything apart, did you notice evidence of water in the runners of the exhaust manifolds or in the valley below the intake? If youre lucky, the water intrusion is from a cracked intake or exhaust manifold (or both). If youre unlucky, you have a cracked block. I am assuming that your freshened heads were magnafluxed and can be ruled out as the source. Im also assuming that you didnt overheat the boat in the short amount of time you had it running.
To figure out which it is, do a compression test on all cylinders. That will tell you if your block is cracked in the area of your cylinders. If theyre all within 10%, then its likely an intake or exhaust manifold issue. Which cylinders have water in them? All of them? Just one bank?