Water is plentiful in the solar system but not so in many places where the Belters live and work. It's expensive bringing water to these places, sometimes from across the solar system.
Water is primarily used for the reaction mass for Epstein drives, teakettle drives and maneuvering thrusters as well as for production of O2 on spacecraft. Ceres must have a lot of water loss, it certainly isn't a closed system though it may have been intended to be in the past.
Other valuable commodities found in the Belt and elsewhere are metals, silicon for ceramics and lithium, which is a major plot point in Book 4, Cibola Burn.
