How much water is there on the Earth?

Depends what you mean by "sustainable".

There's far more fresh water falling on the US than is being "used". Go look at any river. Gazillions of litres of fresh water per minute just flowing out to sea and getting salty. Then imagine the number of rivers doing that.

Of course damming every river and diverting it to other uses is impractical and would have bad effects on various ecosystems, but there's a lot of spare fresh water just running out to sea.

And then remember that the vast majority of the water that we "use" is not getting split into hydrogen and oxygen, it's staying molecular water and it's hanging around.

The best example is the Ogallala Aquifer. "At places, the water table was measured to drop more than five feet (1.5 m) per year at the time of maximum extraction." https://en.wikipedia.org/wiki/Ogallala_Aquifer So it's a huge resource of fresh water that's being drained much faster than it's replacement rate. But, building deeper wells is cheap and there is little value in untapped water so it's reasonable for now.

However, long term it's a finite resource. At the same time a lot of fresh water ends up in the ocean so desalination needs to compete with simply moving water around.

Locally when transport is more expensive than desalination it may be useful, but long term it's a balancing act between moving water a thousand miles+, desalinating locally, or increasing efficiency.

PS: In theory these Aquifers could be used as a cheap form of transport, but when any well get's to tap the water for 'free' who pays for it becomes a major issue.

Groundwater is the resource we are depleting, because everything else runs in a circle. Here is a good worldwide groundwater study, based partly on gravity inversions from the GRACE twin satellites: http://www.jpl.nasa.gov/news/news.php?feature=4626

The conclusion is: Probably the most at-risk aquifer (based on people affected and severity) is in the Gangetic plain in northern India. There are also critically threatened aquifers in the Arabian peninsula, southeast China, and our own Sacramento Valley.

This conclusion is supported by other hydrological modeling groups as well (e.g., the very highly-cited, but older, http://onlinelibrary.wiley.com/doi/10.1029/2010GL044571/full)

Let's melt the whole thing!

But on the other hand, even if every non-floating glacier and snowpack melted and flowed into the ocean, that would only increase its mass by 2%. Combine that with the maximum thermal expansion from -4 C to 50 C, which would increase volume by 1.5% (and kill us all), and the highest the ocean could possibly rise above its current level is about 10m.

The highest storm surge ever recorded was about 15m. The tsunami waves from the destruction of Krakatoa ran up to 50m tall.

So now you are armed with the knowledge you need to scoff haughtily at scientifically inaccurate Hollywood movies. For further reference, 1 WTC is 541m tall, Empire State Building is 443m at the top of the antenna, BoA tower is 366m, Chrysler and Times are both 319m. NYC has more than 200 buildings over 150m. The Statue of Liberty is about 93m.

So when you see that scene in A.I. (2001) where the arm and torch of Liberty barely protrudes above the surface of the ocean, you can say, "Ha! The planet doesn't have that much water on it! Even on an ice-free Earth, and NYC in the heart of a hurricane storm surge the same size as the worst in recorded history, the water wouldn't even reach her big toe!" Waterworld (1995) is right out--the premise is scientifically impossible. You would have to systematically throw comets into Earth capture trajectories with a total mass of about 3x10^21 kg, adding double the current mass of all water on Earth, or about 1/2000 the current mass of the Earth. That's about the same amount as all the water on Europa.