Freshwater isn't going to run out any time soon. In fact, globally, there will likely be more in the future. It's just that its distribution changes, and not necessarily for the better.

As the earth gets warmer, more water evaporates from the oceans. What goes up must come down. Water coming down - yep, that's rain and rain is freshwater. So, as the earth is getting warmer, the supply of freshwater necessarily gets better.

But: as climate changes, the areas where it rains change as well. Also, the times may change. It's no use to have an even wetter rainy season, that'll only wash more fertile topsoil away. It doesn't help if there's more rain elsewhere if dry regions were people live become even dryer. Those are problems of distribution and since freshwater can only be transported so far before it becomes undrinkable (germs multiplying etc.), the only solution might be to move people rather than water. However, the people living where other people might want to move to generally aren't thrilled.

Even worse: If there's more rainglobally, that doesn't necessarily mean that there's more rain on land. There might just be significantly more rain on the oceans and actually less rain on land. People can't move their agriculture to the moddle of the ocean where there's no land.

So, freshwater is not going to run out, far from it. We'll just have to learn to distribute it intelligently, maybe to store it seasonally or to adopt to new pattern of rainfall. De-salination is a way to avoid all of that and keep going as before, but it's costly and requires huge amounts of energy.