There's a huge difference of usability between different water sources. You can't irrigate your orchards with salt water from the ocean. You can't bathe in waste water from most manufacturing plants. You can't drink ground water that has been contaminated by fracking.

There is a finite amount of potable water. There is a finite amount of usable water for agriculture. Pointing at all of the water on earth and acting as if it's all immediately usable without significant, costly barriers is silly.