Yeah I don't think we'll be colonizing it anytime soon. Its simply too cold. The moment we thicken the atmosphere, anything mechanical gets crusted in ice, like those run down soviet towns in Siberia - its only barely doable on earth.

The water is in frozen aquifers. You can see that there is a particarly big aquifer under the Valles Marinaris - the grand canyon on steroids. When water freezes, it expands. You can see that the west side of the canyon is a jagged broken surface, which looks like it got pushed by something underneath it - ice.

I think there's enough frozen water to entirely fill the oceans. There will be 3 oceans - the one that covers most of the northern hemisphere, the Argyre Planitia, and the Hellas Mare.