The water, probably very salty, appears only during the hot season, but life is very stubborn and ingenious, so it is possible (this is my speculation, mind you) that some simple organisms can live on Mars.
This minimal presence of water does not seems of any use for eventual colonization plans, but the existence of extraterrestrial life would be of extreme interest for science.
One of the first questions is, how much biochemistry those eventual bacteria share with terrestrial life?
But in fact the Question Zero would be, is it prudent to organize a mission to take samples of martian life back to Earth? Its behaviour and pathogenic capabilities may be surprising, to use an understatement.