I'm not expert, but I think you're oversimplifying a real problem. My understanding is that we're giving water to farmers at such a cheap price, that they are essentially wasting it on crops that shouldn't be grown where they are growing them. Which costs us ridiculous amounts of water in order to get cheap luxury crops, at the cost of other things. If it were about keeping people fed.. well, that really isn't an issue. We're REALLY efficient at farming enough crops to keep people fed, even without wasting water.

edit: after reading some other comments, it's perhaps inaccurate to say that we're giving agriculture the water. And it sounds more like they bought it years ago at a ridiculously cheap price and are still benefiting from that.