I don't think that the use of water for drinking is a major problem in land use. If you want to save the planet, don't wear cotton. Hemp is much more environmentally friendly. I wonder about synthetic fibres, are they byproducts or do they divert petroleum from use as fuel?

I guess if you really wanted to minimize your own use of water, you could wear a special suit that recycles your sweat and other waste water, I think you could make a crude one with some hefty bags, some duct tape and the filter from a Brita jug.