Perfluorononane is a fluorocarbon with a density of 1.8 g/mL and a melting point of -16⁰ C.
According to this paper it has a vapor pressure of about 0.7 kPa at 15⁰ C, so in regions with low altitude at the northern latitudes of Mars where ice just below the surface is expected, perfluorononane should be mostly solid and when liquid it would barely evaporate away into the atmosphere.
As an example, a small bowl-shaped crater with a depth of 20 m could contain a 14 m deep perfluorononane lake that would have a pressure of about 1 atm at the floor of the crater.
To use as least as possible of the costly substance, an iceberg, that at its bottom would have more or less the same curvature as the bowl-shaped crater, could float on it, with much of its mass below the surface.
The amount of the iceberg above the liquid surface could be adjusted such that between the floor of the crater and the bottom of the iceberg there would be enough space to house a settlement, while the space between the sides of the iceberg and the sides of the crater would make entrances from the surface possible.
Some of the advantages of such a settlement opposed to one in the open air at the surface of Mars would be:
Water (ice) at hand in huge quantities !
The centre of the ice ceiling could have a dome shape that would allow liquid water to float inside it on the heavier fluorocarbon.
Fluorocarbons have good thermal insulation properties and high emissivities, meaning thermal radiation from waste heat of the settlement would be radiated back. That would make it possible to maintain an ambient temperature just below the freezing point of water, opposed to the far below -60° C on the surface at the latitudes mentioned before.
the ambient pressure of the liquid would be 1 atm., so no need for spacesuits to go from one building to another, maybe even no diving equipment for this particular fluorocarbon !
No need for special building material that should protect against micrometeorites and UV radiation.
The buildings would not have to be made airtight at the surface on which they stand because of the equal pressures of the air inside and the ambient liquid.