Trevor, trust me I am well aware of the risks of cranking a no-start engine with sea water being drawn in. I was very careful to avoid water buildup in the muffler and exhaust at all times.
The engine would not start a week after it had run for 4 days, and that could not have been caused by excessive cranking. I did not try to start it in the interim, and when I did I carefully drained the muffler or shut off the raw water intake valve after a few attempts to get it going. And I kept draining it periodically as we tested and tested. If it had filled up with water during that time, the engine would have seized. I have seen engines do that before, and it is almost like having a dead battery. The engine will not even turn over. In my case here, the engine has always cranked just fine but never even coughed after the run up from Florida in June.
My service guy suggested that it could have been a long time in the making (possibly from weak compression) and it could have gotten worse during the trip up from Florida. I ran it fairly hard during the trip after the sea trial problems, which obviously would have increased exhaust back pressure and reduced the exhaust efficiency at higher RPM, especially if the engine compression was not up to par. He suggested, as well, that sitting for a week with any sort of "backwash" water (even heavy vapor if you will) could have caused it to fail. Again, short of tearing the engine down, he cannot say for sure. But we have ruled out every other possibility we can think of, including all those who have offered suggestions and lent support from the group. I am out of options now. There is nothing else to test or try.
I do not want to de-Amel my boat (I have been trying to keep to the design philosophy Henri set in place in all that I do), but I do need to ensure that water does not get into the engine in the future. If that means a bit of rework of the exhaust as I install my new engine, well then I need to do that for my boat. No one and no engineers are correct all the time. And, as you know, each boat is different in how it behaves. I have to deal with what I have. A new engine is expensive but clearly not an undesirable thing..............
Thanks for the support.