re: Ground "water"
Permafrost then, as far as a water source for any cave base.
Although to say "There is NO groundwater on Mars" seems a bit too overly conclusive at this stage. Especially considering what appears to be recent water flows observed by MGS and MRO.
Mars is a big planet and there is so much that has not been explored. Last I checked a few months back Marsis on MCO had only probed a small fraction of Mars thus far.
And I doubt anyone would have said there is liquid water on Encleadus a couple years ago.