I have done three 69~72 hour fasts. In hindsight, doing so without water would have been dangerous and I cannot see any extra benefit.
My only “tactical dehydration” was strapped for 9 hours in the backseat of an F-4 Phantom from Mississippi to Spain. From bedtime to briefing to takeoff, shut down was 24+ hours. I didn’t want to fumble through three layers of clothing to find my minuscule member and fill a rubber “piddle pack,” whilst perched atop an armed ejection seat so I purposefully drank nothing. I was a child of 27. Started my rehydration with an ice cold can of Budweiser after sliding off the wing. That can of Budweiser remains the very best beer I have ever had in my life.