The more you think it through, the more you have to conclude that - as hinted in passing in the original mini-series, or at least the novelisation, it's not really the water that makes Earth useful, it's an intelligent-ish population of 6 billion who can be kidnapped, brainwashed, and used as conscripts in The Leader's wars against The Enemy (which is about as deep as it got about who they actually were :-)).
That would have been an interesting angle to take: perhaps, just as the chemicals the Visitors supposedly wanted turned out to be irrelevant, taking the water was only a means to an end: by causing crop failures the Visitors could force Humanity into refugee camps where they could be controlled? When you look at a lot of history, most recently the recruitment of child soldiers from refugee camps along the Rwandan/Burandan borders, it makes a nasty sort of sense.
An after thought: Possibly, trying to think about the Visitors' plan in terms of scientific credibility is missing the point (waits for someone to point out that trying to think V through seriously is definitely missing the point).
As Christopher points out, you have to see V as an allegory: if you take it as read that, as hinted more in the book than onscreen, that The Leader came to power through a populist movement in a failing democracy where ordinary people are having a hard time becasue of some recent defeat, akin to Weimar Germany, then his plans only have to make sense politically, in the mind of a dictator clinging to power.
It doesn't matter that saving Sirius by stealing water from other planets won't work, what matters is that it shows The Leader has a Great Plan to save the planet, one which requires sacrifice and service from the people (under his control, of course). And as a bonus it even gives the nastier among these ordinary people a chance to lord it over 'lesser peoples' on other worlds.