From a purely "realistic" stand point, water is the least rare thing in the Belt. There are plenty of asteroids with billions of liters of water, there are dwarf planets spewing out into space. Also that means if there is water, there is O<sub>2</sub>, lots of O<sub>2</sub>. Making water and air an issue in the show and books, shows no research AT ALL was done on this topic. Having to travel to Saturn for Water....sigh. It's easy enough to come up with things that could be commodities in a space story. Water and Air and not them.