The various Pegasus engines in UK Harrier variants up until those in the GR7A and GR9A had a JPT limit which when you hit it resulted in an auto reduction of fuel flow and so a cutback in RPM unless you tripped the limiters. So a 50 imp gall tank of plain water (demin ideally to avoid furring up of the injectors) was used. When this was switched on you were in effect pissing on the fire and so reducing hot end metal temps. This gave a lot more thrust – 1000 lb or more depending on the conditions.
When off doing shows etc unsupported one used to fill up the water tank from the local fire engine. The filler cap was on the top of the fuse above the wing and like a car. If asked "Wot you filling it up with water for mister?" the best answer was to look around as if to check for the black Omegas and say quietly. "Its the special fuel pellet I put in first that makes it work"
We needed no antifreeze 'cos the tank was in the engine bay which stayed warm even in a high alt cruise.