Thanks for the clarification on your point. I was not trying to stir anything up and you can probably tell by my membership time and number of posts that I do not poke my nose into a lot stuff on here, especially like some of our northern brethren (i.e. MarylandMark, Masher44). The point I was trying to make was simply that if I had an option of spending an extra $10-$15 grand to get a marine conversion Duramax/outdrive set up that will give me 50% or more longevity between rebuilds and have the same performance it makes sense to me. The reduced fuel consumption would be the icing on the cake and not the reason for the exercise.
The big issue as I see it now with a marine version of the Duramax, Cummings or Powerchoke is trying to developing a transmission or gearbox that will increase the prop speed to those comparable to a gas motor, which would let the diesel operate at RPM levels that will allow it to be throttled and live for a long time. Anyone who has had one of these diesel motors knows they will not live for long at 5000+ RPM regardless of what you have done to the internals.
Hopefully someone will figure it all out and there will in turn be enough demand to make it a cost effective option.