MTU 12V 2000 M94 Cylinder Head Gasket Failure

My guess would be not torqueing down the bolts properly.

It would have been a good idea to check that before unbolting the heads.
I appreciate that if they were directly pulled without doing it, now my suggestion is useless.
But possibly worth considering for the rest, other engine included.

So this has the makings of an educational thread for me. Does it make sense to check the torque on the other heads that may remain in place? Just as an indicator if they are also found deficient.

It would make absolute sense given the circumstances. In fact it would make sense for everyone to do but I've never heard of it happening. I doubt there's any claim to be made on this, but it should serve as a lesson to all to not just live with oil leaks. Had the gasket been changed when the leak first developed that probably would have been the extent of the damage.

Yup, it's reasonable to assume that, but as the old saying goes, hindsight is a great thing...
Pretty sure what happened here is not something anyone would expect from such engine beasts - let alone a 4 years old one with just 800 hours!
And even if as I recall they have single cylinder heads, it ain't a trivial job.

Exactly. As I said it'd be smart for every owner to torque down every bold (and so would be wearing suspenders with your belt) but I've never seen it happen. I don't think an oil leak in 4 years or 800 hours is unusual. It could be from a bolt not torqued down enough or excessive vibration due to something being wrapped on the prop or.... or it could be just a bad gasket or.... That's why that pre-cruise engine room check is so important.

Check the deck of the block and make sure it’s straight. It sure seeems like that ring area in the second photo is raised higher than the rest of the block. Could the engine have gotten really cold at some point in time?

Yep. Bet there'll be a lot of guys buying torque wrenches after today, especially those who've been looking at oil leaks thinking they're not that bad yet.

I was thinking more in the way of a test. If the adjacent heads aren't torqued properly wouldn't that give even more credence to the suspected cause you are all discussing.

If there isn't any factory liability, the OP should check his insurance policy to learn if it covers these types of unforced failures.

T.K. have you owned the boat since new? Is it possible it froze prior to your ownership?

My Mans had to be retorqued, I think it was at 400 hours?

Mmm...
That chap should have thought to do that regardless of your suggestion.
It's his job, not yours.
I can only think of two possible reasons why he didn't, but neither come out in his favor:
1) lack of competence;
2) guilty conscience.

Ref. 2, does MTU possibly prescribe a re-tightening of new engines after the first X hours (50, 100, or whatever)?
If yes, AND they always did the maintenance for you, AND they forgot to do that, I would smell a rat.
Mind, I'm not sure if that's something included in MTU maintenance echelon for new engines.
Maybe someone else can confirm - or not, as appropriate.

Interesting.
Do you possibly remember if that was part of standard MAN maintenance routine, or if it was an initiative of your mechanic?

If correct, that would be the most impressive diagnosis ever made just upon photographic evidence!