A dozen probes, or every probe that was equipped with corresponding sensors, discovered water on Mars or proof of water. Sometimes it's minerals that form only underwater on Earth, sometimes it's a tiny amount of frost in the soil. The most recent news I saw was wide thick water ice, not just some ice containing a bit of water, in a crater of the Northern lowlands - but that was in the general Press.
Whether life can exist there, I doubt it. That's frozen water, made rock-hard by the cold. Ionizing rays won't help neither.
A water stock would help a human crew survive on Mars for two years with technology brought from Earth. Producing breathable oxygen from local atmospheric CO2
isn't a worry and is partly demonstrated without water. But replenishing the water lost at the station, the vehicles... gets much easier if much of concentrated water is available. The cleanliness of water is unimportant for that.
With more water, the crew could grow vegetables locally. That's risky, as all attempts of a closed biosphere on Earth failed. But if replenishing the lost water, and inputting energy to regenerate oxygen, everything gets easier.
Water might hypothetically serve to make on Mars the propellants that bring the astronauts back (keywords "in-situ propellant"). Oxygen can be made from the atmosphere, but not hydrocarbons. Older scenarios (Zubrin's "Mars direct") wanted to bring hydrogen from Earth to make methane locally, I suggested instead to make only oxygen and burn the hydrogen directly: much simpler, safer, bring a bit more hydrogen, bring no chemical plant, avoid the bad yield. But if amounts of water are available, we could bring zero hydrogen, and avoid the hydrocarbon synthesis. One big interrogation is how the energy amount is obtained.
As soon as Mankind has my sunheat rocket enginehttps://www.scienceforums.net/topic/76627-solar-thermal-rocket/?tab=comments#comment-752365
we can bring icy asteroids to Martian orbit the big wayhttps://www.scienceforums.net/topic/76627-solar-thermal-rocket/?tab=comments#comment-757109
or we can make a mission with short trips using launchers like the SLShttp://www.scienceforums.net/topic/83289-manned-mars-mission/?do=findComment&comment=806615
and the better propulsion lets bring enough water and comfort to avoid producing anything locally, which I feel much safer.