Surmount: I know that some older larger radial piston engines had water injection, but I never heard about (I might be wrong) turbine engines with water injection. Some uses fuel to cool the turbine blade (some of the P&W JT9D if remember correct). On the piston engines, they used water to cool the cyl.head/piston of the engine. A pistion engine will have its highest performance slightly below max EGT, but the best fuel to air mix happens at max EGT. So you use the water to cool the proces down to a little below max EGT, and that gives the most HP. Try to ask some DC6 or DC7 drivers, they should know.