I read this story a long time ago - it might have been part of a longer book all set in the same universe (a la I, Robot). I remember it very vividly, but I can't find the name of the story or author now.
The basic plot of the story is that humans have colonized Mars, but it's just the beginning of colonization, maybe 1 or 2 generations of children have been born on Mars. The limiting factor on Mars is the lack of water, and there's a custom where if you go to someone's house, you bring water with you (like bringing wine, I think). They currently get all their water from Earth, but there is some political opposition to this - people thinking that Mars will take all the water from Earth (the Martians make the point that some absurdly small fraction of the ocean like 0.00000001% will cover their water needs for a century, but politicians are making a big deal of it).
In the end, to avoid their dependence upon Earth's water supplies, some Martian colonists take an incredibly risky mission to the Asteroid belt and bring back a frozen water asteroid, showing that they can get their own water if they need.
I'm fairly certain this is not anything too obscure, so presumably someone will get this one quick. Thanks!