Yet another article on martian water, this one from NASA itself.

The article suggests that the recent MOLA camera survey of near-surface water in martian regolith doesn't tell the whole story (see, I told you so) -- there is evidence to suggest occasional catastrophic outflows of liquid surface water. Granted, this has been suggested before, based on MGS photographic evidence of apparently water-carved features, but it was thought to indicate only large outflows many thousands or millions of years in the past. However, judging by interactions of smaller water-carved features with wind-formed dunes, it may be that smaller, swimming-pool-sized outflows occur frequently -- even today.

With efficient recycling technology, a settlement could make great use of the occasional swimming-pool-sized near-surface aquifer. If, that is, they could find it before it drained onto the surface, when it was still easy to harvest.

There is also speculation in the article that Mars' climate cycles through periods of warmth, when temperatures rise sufficiently to melt the carbon dioxide in the polar caps, raise the pressure to around 40 millibars, and thereby provide conditions in which liquid water can exist freely on the martian surface. Combine this with suggestions noted elsewhere that the martian climate may be entering one of these warm phases, and it looks as though we may have chosen to settle the planet at just the right time.