cant you already do that?

Scared of that? But that's all the water! 95~97% of that water is salt water, which is pretty much useless to us. And the 3~5% which is fresh water is not exactly readily accessible.

Only something like 0.5% of the fresh water is on the surface as rivers, lakes, swamps, etc.

Most of all the fresh water, 68%, is frozen in icebergs, icecaps, mountain tops, etc.

30% is buried as groundwater, aquifers or simply soil moisture.

The rest is pretty much in the atmosphere as either vapor (clouds) or precipitation (rain, snow, fog, etc). There's also some water (the percentage is so small it's usually disconsidered) contained in living beings, but the percentage may be relevant in the future, with human superpopulation, we also need superpopulation of food (eadible plants and animals), that may extent over areas that usually don't hold much life.

But none of that is the worst part. We actually lose water everyday from atmosphere to the space, losing hydrogen (and other gases, it's know as planetary wind). But that's also not the worst part.

The worst part is that the hotter it gets down here (global warming), the more gases escape to space, the more frozen fresh water get mixed straight in the salt water and the less precipitation we have to refill the readily accessible sources.

Now you can be scared.