Water, on Mars as on Earth, is essential to life and therefore to any Mars settlement. Water infrastructures, including water treatment facilities and distribution infrastructure will be part of any Mars settlement from day one. Indeed it will be part of the vehicle bringing the settlers to Mars.
Melting, crushing, transportation
Water treatment plants
Extracted water may contain contaminants such as dust, salts and other materials
Potable water may need biological treatment (chlorine).
Water may (will) be mixed with re-used water. There will be large amounts of water removed by dehumidification from greenhouses.
Water will be used and re-created by the Sabatier process. Water may also be created by steel making and aluminium production if these use hydrogen to reduce the ore.