What if there are living things in that water? It would be a crime beyond extant legal conception to inflict on extraterrestrial life present commercial and industrial extractive mechanisms.
We'll never know until it's too late, however, as unless we actually see living Martian organisms (and can recognize them as alive) we can only guess based on such data as we can acquire.
Some years ago Russia drilled to within a few feet of an Antarctic lake that had been sealed off from the surface for (presumably) millions of years. The reason they didn't just drill right in is that we cannot prevent living things from hitchhiking on our equipment, not to mention various other impacts our industrial methods would have on such an ecosystem.
Presently we have no certainty that there is native life on Mars. However, we didn't know there was a massive submartian lake either. IMHO, if we are going to act in a lawful and just way regarding potential life on Mars, we need to study far more carefully prior to digging latrines there.
We have already delivered living creatures to Mars. NASA cannot clean our probes of all life, and we have landed probes. There is no doubt at all that Tardigrades presently live on Mars.
There's a pic of the first Martian we can be certain is there.