Drinking water only when you feel thirsty is incorrect. The feeling of thirst comes as a signal to prevent more severe dehydration. You should supply water to your organism regularly throughout the day, especially after you lose some of it (such as after a walk in hot weather or exercise). You should also hydrate when you wake up, as water levels in organism drop during sleep and doesn’t get replenishment throughout the night.
Your body is not adapted to store more water than it needs, and this is why you need to remember to drink regularly. Full hydration does not happen immediately. Even after consuming a lot of water, the body would need hours if not days to restore its reserves fully. The exact time can vary based on the seriousness of dehydration and which tissues exactly lost the most of water. Skin and muscle tissues are built primarily from water, so when these get dehydrated, it takes your body the longest to restore normal fluid levels.
Water is responsible for a range of structural (transporting nutrient, hormones, and oxygen), cleansing (from toxins and metabolism by-products), chemical (metabolic) and regulating (body temperature) functions in the body. You really need water to live! So now, go and drink a glass of it!