Water

To survive you need water. As time goes by you will get thirsty, this is indicated by the water drop in the lower right corner.

If the drop empties completely you will start losing health until you die.

To combat thirst you need to either eat food that contains water or drink water directly.

Drinking salt water from the Sea is not recommend, neither is drinking untreated water from the stream. In the later case it needs to be boiled in an appropriate container by a Fireplace or on an Advanced Fireplace