Thirst is the blue bar on your interface which is labeled water. If the your water is low the player becomes thirsty and requires water. To replenish your thirst, drink from a tile that is emerged underwater or drink a liquid-based food. Thirst affects the rate at which your stamina regenerates. Higher levels obviously cause your stamina to regenerate faster. You will not die if your water reaches 0%. It takes 2.2kg of water to refill a bar from 0%.