The Sun has a mass of around a third of a million Earths. Stealing a figure from MadSci.org, the mass of water on the Earth is 1/4400 the total mass. We'll say we need enough water to completely douse every atom in that fusion-burning puppy we call Sol, so we'll need... 4400 * 0.3 * a million Earths.
This works out neatly to just over a billion Earths, to get enough water to douse the Sun.
Now, how many Earths are there in the universe? I could try to approximate, assuming 10% of stars are Sun-like and some fraction of those stars will have planetary systems and some fraction of those will have watery planets. But it turns out we don't need to.
There are perhaps 100 billion galaxies in the universe (and let's not quibble over 'visible' versus 'entire' universe here). And each galaxy has maybe 100 billion stars. So as long as you accept that there's at least one watery planet per galaxy-- one watery planet for every 100 billion stars-- we're set. We can quench the Sun with water 100 times over.
The universe is just that big. Big, big, big. The calculation is similar to Drake's Equation for guessing how many E.T. intelligences are in the universe. The Drake Equation also plays with estimates on how many Earth-like planets, then goes further with guesses on how many develop intelligence, etc. It also usually returns the same result-- the universe is so enormously big, yes there's probably anything reasonable you want in it.
After that, both become mere problems of logistics. Even though the universe has enough water to quench the Sun, we don't have a delivery system to get it here. Being big means very little is actually local. So I guess I have to cancel my latest mad scientist plan. Our Sun is safe after all.
That's our lesson for today: planets are way bigger than people, stars are way bigger than planets, and galaxies are way bigger than stars. There's always something bigger than yourself... unless you're a universe. Then, I guess, you are the biggest.
Alex, the Daytime Astronomer
The Daytime Astronomer, Tues&Fri here, via RSS feed, and twitter @skyday