Water is not the problem. More rain falls down each year than the entire human population could ever drink (in a year, that is). To get clean water is the trick.
The big problem is that many countries are not advanced enough to support their population properly. "Properly" meaning decent health care facilities, clean drinking water, plenty of food, a good diversity of food, proper infrastructure, electricity, higher education, enforcement of the UDHR, etc. FOR EVERYONE (not just the rich or powerful). If all nations were up to this task, instead of fighting each other or their own population; we wouldn't be bothered by the sheer overcrowding of our planet.
The next problem is space. Would all 6 billion people live at the standard of most Western civilized countries, there wouldn't be enough food to go around. We'd have to cultivate the Sahara desert as well as Saudi Arabia in order to feed everyone. But I wonder what the function of so many people is...
If 6 billion (or however many of us there are nowadays) people is just a statement to ourselves that we have the power to sustain such a number, I'd be rather disappointed. If those 6 billion were all great individuals I'd say it's a different matter, but it doesn't take 6 billion humans to keep things running, discover new stuff, explore the universe and create art (<= basically all that needs to be done).
A mere 2 billion would be plenty, I think. A number much easier to sustain, especially considering our fossil fuel consumption; leaving much more space for big backyards or just real nature. And we wouldn't miss out on anything good, I think...
Looking at other planets - and outside our solar system - will be crucial to mankind's survival. Especially when our earth destroys itself, or our sun starts blowing up in our face.