If the
fuel cost is a factor in your decision, you should probably consider not
buying a boat.
Difficult to give answer without more information. Specific vessel, engines, desired speed , etc. all have a bearing on the number. At max it could be 0.8 nmpg, at 6 kts it could be 3 nmpg.
With the average
power boat running around in the 100 to 150 hrs per season, the fuel cost is not a significant factor when you consider depreciation, loss of investment opportunity,
insurance,
storage,
maintenance, etc. Just
insurance alone costs me 4 times my fuel cost.
Someone once told me “A wise man never works out his overall cost per nmpg and if he does, he never tells his wife.”