Consider the many organs and systems in your body that need water:

brain

tissues

cells

blood

skin

microbiome

colon

These are just some of your water receptors

Consider that you expel fluid (not just water) all the time, and not just when you need to urinate or have a bowel movement. You also use fluid when you sweat, when you breathe, when you cough, when you sneeze. If you are only drinking a little water, daily, how will your body get the adequate fluid that it needs?