I wrote a book review on Amazon:
There is roughly 326 million cubic miles (1.34 billion cubic km) of water on the Earth. The original aliens brought 50 motherships that were about 1 mile in diameter each. Individual ships could hold, at maximum, 1/20th of a cubic mile of water (assuming space needed for cargo holds, shuttle bays, engines, quarters and whatnot). Even if the aliens sent an extra MILLION ships (and that's a HUGE exaggeration) to the Earth after the original first series, they would ONLY have taken 50,000 cubic miles of water on that visit alone. While this is, indeed, a lot of water and would have caused notable climate shifts, it would not be more than a drop in the bucket in comparison to the rest of the oceans.
[SNIP - LIMITING THE QUOTE]
Thank you for that - while on the daily commute, I tried to work out some 'back of the envelope' figures for how improbable the water thing was when this came up here a few months ago, but gave up as I didn't have the necessary figures. Thanks for working it out, as it really brings the point home: the Visitors could steal enough of our water to damage our eco-system, but they couldn't possibly steal enough to save their own eco-system.