The Pegasus engine in the Harrier (all versions) did indeed use demin water injection. It had 50 gallons (proper ones) and lasted c 90 secs (or c 3 mins with half flow mod), giving an extra 1500 lbs of thrust. Extremely useful in hot/high conditions as it reduced the JPT considerably for given rpm. I used to give a quick blip as the jet ran up the ramp (for mum!) which left loads for subsequent landing.
Very useful in the Farley Climb as the JPT could otherwise get pretty high with high rpm and low forward speed. It made the exhaust quite smoky and that was a bit of a give-away in a low-speed fight 😊
it was fun to turn up at a display and when asked how much fuel you needed, reply “Fine for gas thanks, just give me 50 galls of water!”
Mog