All the Mars models I'm familiar with posit a planetary heat flow that creates a liquid water CAPABLE zone between a few hundred meters of the surface and the solid bedrock several km down. Water in mass quantities could be percolating through this zone as it does on Earth. And as on Earth, potential biological processes could care less about the temperature, pressure, and radiation flux of the surface -- where the miscellaneous scum can claw out a toehold. That being said, I agree that there is a substantial chance that the morphological evidence we currently attribute to surface water activity is misniterpreted (i.e., where are the carbonates??) -- but if I had to guess, I'd go with the flow -- of water, lots of it, and even now, underground.

But as Saint Carl often said, we live in that one unique blessed moment in human history where we can speculate, and then actually find out.