I have a Yamaha 200 with about 230 hours on our 2002 21SE CC that I bought the boat 9 months ago. As RiffraWillie mentioned, it's efficient. I run it fairly hard and the computer reads that I get about 3.75 MPG. I'm not sure how accurate that reading is but I'm happy with the economy and the previous owner that repowered it was very satisfied with the upgrade.
I have not done extensive logging with winds, speeds, waves, but I'd say that typically if I look at mileage, from my gauges, I see roughly 4 mpg give or take.