You suspected carbon build-up initially thus the steam cleaning. Maybe the water vapor caused localized cooling of the valve guide causing it to shrink just a hair making the carboned-up exh valve stick open??

On the brighter side, looks like you'll have a chance to address #7 while you have the head off.

A pressure test is in order my friend.

On the bright side, you may have inadvertently discovered a huge problem that left unchecked could have caused a catastrophic destruction of your engine. Leaky headers have destroyed many thus the design change.

you got lucky that piece of guide plate didn't go down under............

If you had enough cylinder pressure your piston rod can look like your pushrod.

If I pull cylinder head this weekend, and the tops of the pistons look fine, what's the chances that this is purely a top end problem and the bottom end is fine?

Let me get this right... You poured water down the PCV hose? You did not mist into the throttle body?

"poured" is a strong word.

"Trickled" into the hose would be a more accurate word.

I've done this before on other engines (including the starboard one 5 minutes earlier) with no ill effects.

Pending final analysis of what went wrong, I reserve the right to re-evaluate my choices

You and Eddie sure have bad luck when it comes to boat motors.

Yeah, I've noticed that. Though, I can't really get too upset, I bought this thing with no real knowledge of how/when/if these motors had any real work done to them in the last 20 years. Best I can tell, the motors were redone in 2003-2004 with no evidence of being opened up since, and somewhere around 450-500 hours on them since then.

Even if I had the leakdowns done prior to purchase I don't know that it would have been bad enough to make me not buy it.

When I bought the Cigarette, I bought it with the knowledge that the motors were recently built, and got a false sense of security about the likely reliability of what turned out to be very sub-par builds.