The answer is we need to quit fossil fuels and reduce our numbers to something comfortable and sustainable.
As a civilization we might not make it through this century so it's pointless to imagine we humans will make it out of the Solar System.
There are a couple of options. We can keep burning fossil fuels until climate changes crash this civilization, we can accept nuclear power and continue to live as we are now living until some other natural system supporting us collapses, or we can accept a much slower paced lifestyle powered by non-nuclear, non fossil fuel sources. My own choice would be the last -- a lower energy lifestyle and an end to economic "productivity" as we now know it.
I enjoy Star Trek, I read plenty of Science Fiction, most of my education has been in the sciences, I think the internet, cell phones, and other technologies are magnificent things, but I don't think humans, such as we are, will ever leave this planet in any significant numbers. Perhaps our intellectual descendants will, "people" of some sort better adapted to space travel (robots, artificial humans, who knows, the sorts of creatures that can walk around on Mars naked...) but it's more likely, if we continue down the path we are on, that our species will join all the others that have become extinct.
In my "optimistic" crystal ball we've created a political system where we can gracefully relocate populations from places that have become hostile (rising sea levels, water shortages...) everyone is literate and has easy access to further education and the worldwide communication system, and we've lost our appetite for "factory farm" meat and personal automobiles. Space programs are very active, using increasingly sophisticated robots and instruments. (Thrill seeking humans are not excluded from space, but human space travel is not the focus of public research.) Birth control is voluntary but almost universally practiced. Women are educated and empowered. Medicine is still advancing rapidly and medical research is well-funded by government institutions, with new drugs and treatments released for the benefit of all mankind under non-proprietary share-and-share alike. licenses.
Desalinization is essentially a mature technology today, but the problem is not "lack of water," it's human ignorance and lack of any awareness (or denial) of the actual causes of our problems.
You can't bolt a desalinization plant on a fundamentally broken and unsustainable economic system and expect it to fix things.
= new reply since forum marked as read