I've always claimed I got 5 mpg in my homebuilt 20' Standard Tolman skiff but to day I measured it over a wide range of speeds using the GPS as my knotmeter and the fuel flow meter. With 4 adults aboard on a plane she gets over 5 mpg, roughly 5.2 mpg. Maximum speed with 4 adults (90 hp 2 cycle Yamaha engine)was 39 mph.
Then one my son's friends wanted to try wakeboarding from it. I had never even considered such but we rigged a tow bridle and it worked very well.