There is a very easy way to save fuel on a power boat - throttle back.
A displacement hull should be fairly fuel efficient to begin with?
Carl is right, eliminating turbos would be a very costly undertaking. I believe in many applications for a given amount of horsepower forced induction will be more fuel efficient. A turbo requires much less power to drive than a belt driven supercharger - turbo wins hands down for a given power level and boost required to achieve it, since parasitic loss of HP to drive the supercharger is significant.
On a forced induction application the higher levels of boost are where more fuel is required. Depending upon your application, if you are not spooling up the turbo(s) and creating boost, fuel efficiency will be no different than a normally aspirated application. Some applications also have sequential turbos (not sure about diesels in boats) in which case again, keeping the RPM's below the point the secong turbo kicks in will save fuel.
Just for kicks, I would check out fuel consumption (GPH) on a normally aspirated engine in your target HP range vs. your turbo application before I'd seriously consider changing anything. If I had to hazard a gues, the turbo will be more fuel efficient for the given HP.
John - aka poster child for fuel inefficiency
