Donual,

Not a lot of water in space. What is there, one would want to capture. Recycling is fine, but there are always some losses.

Cheapest way to terraform Mars would be to take chunks of ice from Saturn's rings and put them into collision orbits with Mars. That, plus an effort to break up the mineral oxcides to get free oxygen,, and we could have a number of good sites to live in (say, in canyons or depressions where the atmospheric density would get up to what people live at in the Andes or Himalayans)