"Would the water have all evaporated into space?" According to the paper at this link, water is disassociated into H, H2, and O by the UV light that reaches the surface. These lighter species can reach escape velocity because of Mars' reduced gravity.
It's an interesting paper.
A UV process would take a long time. There is still water on the surface: mostly in the polar caps but a trace still in the atmosphere. Sure hope they find substantial subsurface water because hauling water that far is going to be quite a chore.