I want to add, that until quite recently it was believed that Mars is a dry desert except for water at the poles. But recently abundant water has been found almost everywhere in the form of ice. Amounts up to 50,000km³ of pure water ice in one location and another area with a water volume equal to the North Sea. The british, german and scandinavian members of this forum will acknowledge that's a lot of water.
The total known water on Mars would be enough to cover the whole surface with app. 20m of water.