There is no drinkable water on Mars. It's filled with brine, which is just another word for salt water. So, I hope you enjoy drinking salt for breakfast, lunch, and dinner. Tasty.
Well, if that is the only problem, then it's a no-brainer. Modern filter systems (reverse osmosis, nanofiltration) work on a level where you can remove salt rather trivially. It's still a tidbit expensive for home use (although you can afford it if you really mean to drink pure water -- yank out some fifty, sixty grand and you're sold!) but compared to the cost for making a trip to Mars, this would be insignificant.
Also note that distillation, which is energy intensive but an otherwise trivial and cheap process to remove salt (energy is abundant that close to the sun!) is a process that has been known to man for centuries.
I really see radiation and lack of atmosphere as much more limiting factors than a little salt. That, and the black ooze left behind by the Engineers.
