we're not going to run out of water for human consumption. some areas have drained their water tables visibly, so that the water tables are going down faster than they're being replenished. (but last i heard, the consumption/refill rate has levelled off in those areas..)
anyways, as water levels go down, water gets more expensive. (duh) so in areas where water is more expensive, it's less advantageous to grow water-heavy crops. production of those crops will move to a more advantageous area. that's about the only real effect.
TL;DR:
we're never going to get to a point where you can't get a glass of water. worst comes to worst, you *can* get the equipment and filter it yourself.