Why do you think that you need to ingest water in order to have water available?
How does a camel do it? The necessary water for long periods is stored in fat reserves.
the human body is around 70% water. That's about 8.75 gallons or about 35 liters per 100 lb body weight.
Now obviously not all of that is available for consumption, but there is certainly an excess and the body does know how to conserve its reserves during a fast.