Let's talk MPG.
What boater does that? Isn't that masochistic?
I prefer the perspective that it's perilous to NOT know how much fuel you'll be burning. I mean, when you plan any vacation, do you not consider the value you are getting for the amount you're spending? I do. This is why I resent paying more than $100/night at a hotel (which is getting more and more difficult, of course).
Back in college I bought a diesel Rabbit. It was a great car. Lots of fun to drive, and very reliable. I had a job delivering pizzas at Dominos, and they paid me by the mile, so with that and tips, I was making good $$. So when I started looking for what turned out to be the Contessa, I ran into a Chris Craft Cavalier with twin 350 Crusaders. When I asked the owner about fuel usage, he replied, "this boat isn't for you." Well, he couldn't have been more right!
When I narrowed my boat choices down to the Contessa, I quickly learned that depending on the year, you can get a sterndrive made by Volvo (82-87), OMC (87.5-89) and Mercruiser (90-newer). The Volvo has the reputation for being the most efficient, and most reliable of the three, and the mileage difference between the three is impressive. At 21kts, we are talking 9 vs 12 vs 14 gallons per hour! So, not only does my boat have the efficient Volvo drive, it's also a duo-prop unit and the engine is fuel injected. This combination yields just under 7 gph, or more than three miles per gallon! Match that anywhere - I dare ya!
Okay, enough gloating. So when I look at these bigger liveaboards, MPG is important to me. Of course, the first thing I realize is that I'll need to slow down. A displacement hull is likely in my future, though some boats, like the soft-chine 48' Tollycraft, might give me the option of great economy at displacement speeds, and still reasonable economy at 10-13kts. We'll see. But this is one of the first things I'm looking for...