Side note: Hellas Basin.
I believe that the highest pressure on Mars is between 10 and 12 millibars. his is deep in the Hellas Basin. Under the right conditions, water can exists as a liquid under these pressures and those conditions are sometimes present in Hellas.
In Mars Dry air, water would evaporate at a high rate. If enough were present and it was re-supplied, you could get a large body of water in Hellas. Also, the soil there could be wet when the temperatures are right.
Elsewhere on Mars, underground water could flow on there surface for a long time if it was in a large volume. A protective ice cap may prolong how long it stays liquid. But small concentrations, like what these people are pushing, would evaporate quickly.