Overall water usage not just irragation?
I would say cow meat. Not only do you need water to grow their food, but the animals drink water as well, and you need water to clean their stables. Otherwise, fish. They need a bloody ocean.
Least water: You can eat Prickly Pear, which is a cactus: http://www.amazon.com/Cheris-Prickly-Pear-Cactus-Jelly/dp/B000BHH3M0
Most water: cow