There are no real gains from growing corn for fuel. There is, however, a large cost of water consumed. Whether the land were simply to lie fallow, without irrigation, or whether less water-thirsty crops were grown (corn ethanol requires over 1,500 gallons of water, for every gallon of ethanol produced), there would be still be plenty of crops for food. There are plenty of alternative fuels — natural gas from fracking being the most obvious.
On my fields, I can generally count on receiving about 1 Million gallons of rainfall per acre, per year. Even at 200 bushels of corn per acre, that comes out to 5,000 gallons of water available to grow each bushel of corn (and each bushel of corn will, in turn, yield more than 2.8 gallons of ethanol). Even so, corn which is used as ethanol feedstock, is still available as animal feed. Only the starch is removed, ruminants don't benefit all that much from the starch in corn anyway, and nobody really suggests that the average American needs all that much more corn sugar in his diet.
The fact is that the rain which falls on my land is mine. If I want to use it to grow corn, that's my business. If someone else wants to use his water to have a nice big pretty yard, and not produce a damned thing of value, that would be his business.
What's wrong with fuel ethanol in the US isn't that corn uses too much water, it's that the central government has chosen to screw with the market (the same thing that's wrong with electric cars, passenger trains, letter delivery, education, medical care, ad infinitum).