fuel economy

I do a lot of open ocean tours and just completed a trip from Sorrento to Refuge Cove Wilson's Prom return (460klm), seas were rough, average speed 45 kph, rpm 5k to 5.5k as in the air a lot due to conditions rpms continuously jumped up and down to 8k. Ski was loaded with additional 80ltr fuel...

I recently purchased a 2001 Islandia with the Mercury 240 Jet Drive. I am very pleased with the boat, but not with the engine. I do a good bit of Intercoastal waterway boating and this boat isn't practical at all due to the terrible fuel mileage. Anybody out there know what kind of fuel economy...