It all depends on the engine and the boat. You'll probably only get a mile or two to the gallon, but then you're not driving it around like a car. You then also have to consider that people regularly drive around in vehicles on a daily basis that aren't all that more fuel efficient.
If you actually care that much, you could always spend a bit more money and go solar. Boats is one of the few vehicles where solar power works well and has proven itself (for example, the first usable solar boat is thought to have been made in 1975). You also don't have to go just solar, boats have been powered by wind for centuries and the water generally has much stronger wind than land anyway.
I wouldn't doubt that it would be easy to go green and make your fuel cost zero.
Mine goes roughly 2 kilometers on a liter of diesel, which is pretty cheap by boating standards. The reason is that the engine isn't terribly large, and thus cruising speed is around 7 miles an hour. If I wanted to go 30 miles an hour my fuel consumption would at least quadruple.
And a house isn't a hole in the land you pour money into?
Acceptable home maintenance costs are between 1 and 4% of the house value, however this doesn't necessarily extend toward the higher home prices.
Of course all these prices vary, some people pay ridiculous amounts for home maintenance because they can't do simple tasks and some people pay almost nothing because they can do everything themselves. However, I'm sure the same goes for boat maintenance too.
It's the mere fact that paying $4,000 in home maintenance of a house costing $250,000, and paying $4,000 in boat maintenance on a boat costing $50,000 that you live on.
I suppose your food costs would be significantly cheaper if you like fish though. Well if you're a good fisher. Personally if I had to rely on my fishing skills I'd probably die from blood loss.
