Personally, I don't see how liquid water in any significant amount, can exist on the surface of mars.
The air pressure is too low. If you put a bathtub of it out, it would be gone in less than an hour.
True. As demonstrated in physics class, the water will either boil off or freeze or both right away, a function more of atmospheric pressure than of temperature.
Today's chemistry lesson - Phase diagram...
http://www.chemistrycoach.com/Phase_diagram.htm