Well, I don't really know what I'm talking about, but at the end of this report, it says that water consumption was 2.1 gallons/minute in high-power flight.
See supplement Water Injection on P-47D Airplanes, 3 February 1944 that can be found at the bottom of the test report at
About halfway down, it says that such planes carried 15 gallons of water in tests. I don't know if that was typical for combat missions.
"To report results of flight tests on P-47D-10 airplane, AAF No. 43-75035 run at the manufacturer's plant. Airplane equipped with Pratt & Whitney R-2800-63 engine with water injection; standard Curtiss 714-1C2-12 propeller; type A-17 turbo regulator. Airplane ballasted to simulate the following conditions: Six .50 caliber guns; 300 rounds per gun; 305 gallons gasoline; 15 gallons water; 14 pounds pyrotechnics. In this condition the gross weight was 13,234 pounds with a C.G. wheels up, of 29.63 percent M.A.C. Mixture auto-rich; throttle wide open on all tests. Horsepower data obtained with torquemeter."
I'm sure Rooster can clear this all up.