Water
From Galactic Fleets Wiki
Water was used to keep your population alive, but an update changed it.
Now, water is used to produce new civilians, both food and water is used to produce civilians. It takes 1 food and 1 water to create a new civilian, without either food or water, civilians can't be born. Example, if you have 250 food and 500 water, 250 civilians will be born while the other half dies.
Water is created from the planet page. Click on work and have how much you want it to be produced. The amount created depends on workers, planet and reservoir or Bio Dome. Reservoirs and Bio Dome increases food/water production.