All life is based on large quantities of Water. The same will be true on Mars. There has to be a major and reliable source of water on Mars.
What options are there? I read an interesting article yesterday that said "Our results show a two-order-of-magnitude diurnal variation of water vapor pressure, suggesting a strong atmosphere-regolith interchange", in other words, the soil on Mars extracts water out of the atmosphere in the nighttime and releases it in the daytime. This means that if we collect the soil and "bake" it, it would release water vapor in a controlled environment. We could then condense that water vapor to get useful/useable water.
I think you need to compress the air and cool it and you get condensation. I think the advantage of baking the soil would be that the concentration of water vapor would be higher so less energy.
Hmm... maybe the smartest way would be to just build greenhouses and pump compressed air into it. Plants grow, absorb CO2, release oxygen and water. At night you lower insulation and condense excess water out of the air. What is left is a surplus of oxygen. Of course that requires a ton of glass.