Theoretically, any planet that has water could sustain life in some form. However, the temperatures on Mars are very extreme. Humans would have to find a way to protect themselves from extreme desert like temperatures during the day and subzero-arctic temperatures at night. Water alone won't take care of us.

The theory that Mars can sustain life is up for discussion, as water has been detected through data collection. However, the surface of Mars contains a lot of salt, which is not conductive to sustaining life. You can find more information here: http://www.abc.net.au/science/articles/2...