In battlestar galactica season 1 episode 2 their watertanks were sabotaged and they lost
Almost 60% of total potable water reserves.
The rest of the episode were them looking for more water. Their supplies were announced to last for a very limited amount of time. (A week or two?)
But earlier in the episode, it was stated that
Galactica's water recycling system is close to 100% effective. For all intents and purposes, there's not a drop of water that's wasted aboard.
ADAMA: We have enough water for several years before replenishing.
What could possibly consume the rest of the water in such a short period of time?