Another factor to consider is that the diesel boat will likely cruise at a higher speed. Another real world example of my last boat, a 36' Carver with 8.1s an my current boat with diesels:
The old boat cruised at 23 knots and burned (on average as measured by fuel consumed / hours ran) 26 GPH.
The new boat cruises at 26 knots and burns (by the exact same measure) 21 GPH.
I prefer to look at this on a dollars per mile basis because we don't wake up and say "let's run the boat an hour today" we say let's go to Annapolis today. The number of miles to Annapolis doesn't change based upon GPH, MPG or knots anything else.
The old boat at 23 knots and 26 GPH and an average fuel price (based upon fuel log from last year) of $3.50, translates into a $/nm of $3.95 [(26/23)*3.50=3.95]
And the new boat at 26 knots and 21 GPH and an average fuel price of $2.50 (based upon this year's fuel log), translates into a $/nm of $2.02 [(21/26)*2.50=2.02]
So, basically it costs us almost half as much to run this bigger boat at a higher speed. As I tell me wife... the more we run, the more we save.
P.S., I am also not "anti-gas", I'm just providing the math.