You generally feel like trash
You know that feeling when you have some sort of indeterminate illness that knocks you out for a few days, and you just call it a cold or flu? Lack of water might just be a cause of some of those symptoms. Headaches and sleepiness. Achiness, lethargy, and general grossness. That's all because blood volume and blood pressure drop when you're dehydrated, then muscle and nerve functions slow, and thus, fatigue sets in.
According to Dr. Hanson Lenyoun, "Dehydration can negatively impact your physical and mental performance. Impacts have been seen on endurance, recovery time, motivation, mood, attention, memory, energy levels, and even skin appearance and immune system function." Basically, every system in your body depends on water, so it makes sense that you could feel like a rotting compost heap if you don't drink enough.