Or he means literally 3D-print structures using water, which will freeze in the Martian atmosphere to form water ice - which is actually a pretty great structural material if used appropriately.
Unfortunately, exposed ice will sublimate pretty quickly. You could possibly use it as a bulk structural material when covered, and certainly for radiation shielding.
You can also use water for holding your pressurized structures together: bolt some pipes to your structure, bury them, pour liquid water through the pipes and into the soil, let it freeze, and you've got anchors made of ice.
Aye, true that. Well, cover it in mylar, and it was rads I was thinking of predominantly. You could potentially make something like http://en.wikipedia.org/wiki/Pykrete using martian topsoil.
