Water is a liquid resource formed from melted Ice. It is part of no tier's makeup: all of it was ice first.
Formation
Water forms by Melting only on bodies with a gravity of 500 or more, when they are warmer than 273 K: close to their star, or warmed by an impact. Below 273 K it turns back into ice by Freezing. On a body with a lower gravity it boils off above 373 K. Water is lighter than rock, so when a molten body settles into layers, it rises to the surface as an ocean.
Uses
A Smelter also melts ice into water, and a body with an ocean can be mined for water directly. An Electrolyser splits water into Hydrogen and Oxygen, and a Mixer blends it with polymer into coolant.
Handling
As a liquid, water goes aboard only a tanker or the hull, though any ship can unload water it mined.