I have a boat like that with a similar powertrain. I get 1.3 nautical
mpg at best on plane (20 knots) and 2.3 nmpg at best at hull speed (6
knots). I use this boat 150 to 200 hours a year (mostly at 6 knots) and
buy a bit of gasoline. If/when I repower, I will probably go back to gas
because the motor is so much cheaper, it will bolt right on without any
modifications, it's quieter and I can buy a lot of gas for the
difference in price. ($10,000)

One issue for me is range, which would be better with the diesel, but I
have an auxiliary tank.

Capt. Jeff