Water is a block in Lunacraft that makes up the Ice Lakes throughout your moon.
This material is called "water" internally and in the crafting guide; however, the scanner shows it is Hydrogen Fluoride. Because this substance only freezes at temperatures below -84°C (-118°F), we know the moon is very cold.
The block makes up the Ice Lakes, which can be around 20 blocks deep with the moons Terrain Roughness setting set to max. Because Water is not an opaque block, and ores seem to be fairly common in Ice Lakes, mining ores here is easy.