The problem with “Water” was that it seems that water is a scarce resource in the universe. But if you find some, you’re good for 4+ years....
If you remember the episode, Galactica had advanced recycling capabilities, and it was completely self sufficient for their own use. The problem was that they also had to supply a number of ships that weren't. Plus, don't forget that over time, they lost more than 20% of the "fleet," which would make the remaining water last longer.
Mark