Much easier! No one is fighting for the water, and it doesn't have to be lifted up from a planet. Why fight a war for the privilege of lifting all that mass out of a gravity well when you can hoover it up and fly away?

Also, when I said the nebula were more massive than the entire solar system, I couldn't remember if it was hundreds of times more, or thousands, or millions. I just remembered it being astronomically more than the amount of water on earth.