Thst depends on how deep the water is.

There are two problems:

The high iron content of the Martian crust means that it is likely to obscure radar waves and limit their penetration. There was a lot of discussion on this topic at last year's GeoMars conference and the radar enthusiasts modified their stance somewhat in the face of determined discussion by several geophysicists including Gary Olhoeft and Bob Grimm.

Essentially the "several km" of penetration is a maximum, not a mean. However, there may well be areas where penetration is better, especially in the polecaps themselves.

The second problem is that if Mars is as "cold" as I believe it is, then the depth to liquid water will be much greater than expected anyway, putting those interfaces beyond reach. By "cold" I mean the gothermal heat flux, not the actual surface temperature.

However, I look forward with enthusiasm to any new dataset for Mars, and hope that some interesting "hot spots" get discovered.