Mark generated his own water, by burning Hydrazine (N2H4). In the film, this initially caused a minor explosion... I can't recall if the same thing happened in the book. Even if all the water he had gathered boiled/sublimated, he could probably generate more, assuming he still had hydrazine and LOX.
Hydrazine is used as rocket fuel, and may have been used in the MDV which remained on Mars.
It's probably worth pointing out that the water he generated wasn't intended as drinking water for him; it was needed to grow his potatoes. With all the plants dead, he wouldn't need as large a supply. He probably could have recovered enough for his own needs as detailed in Cody's answer.
With all the plants dead, he wouldn't need as large a supply. He probably could have recovered enough for his own needs as detailed in Cody's answer.