Thanks for all your replies! I'll try to answer a few more questions.

The boat currently is an inboard/outboard which I will be replacing with a bravo unit (I'm considering the Bravo III but I'm concerned with their corrosion problem).

The boat is used for a number of tasks, scuba diving, water skiing, fishing, and occassionally when weather permits, long distance expeditions.

Fuel range is a big problem, When traveling to some prime scuba diving spots, I have to carry an additional 100 gallons of unleaded in 55 gallon drums on deck to make the trip.

I was reading some articles about some diesel engines mercedes produced for prototpyes that were essentially modified 617 engines with larger injectors and bigger turbo, which put out 190 horspower.

As far as the swap goes, just about anything will fit in the engine compartment provided the engine isn't too tall of heavy (leaves out the cummins 5.9), there is no transmission to bolt up to, only a bell housing which appears to have been hacked from a chunk of aluminium.

I's like to limit the cost of the engine replacement to 5000 or less, as the new outdrive is going to cost me that!

Thanks for all your comments, I've definatly got some thinking to do.