Actually its more complicated than that - though turning off the water caused the loss of the crops that were growing, and contributed to some initial looting, things got better for a while - here is a link to an article in the New York Times in November 2005 Gaza Gets Ready for a Harvest of Produce and Promise

But Israel blocked any exports of the produce and very effectively wrecked the attempt to set up an export business to Europe which could have made an enormous difference for Gaza. This meant the greenhouses had no possibility of being run commercially - they couldn't have covered the costs of production.