Water is an essential part of you. Actually, it's most of you.
Up to 60% of the human adult body is water. Among other functions, water is essential to smooth digestion and excretion, oxygen flow throughout the body (including the brain), temperature regulation, and tissue and joint lubrication.
Are you thirsty? How do you know?
Too little and you might:
feel unsatisfied or un-full
lack of muscle coordination and balance
feel brain fog or slow thinking
feel sluggish or fatigued
not recover efficiently from physical activity
Too much and you might: