Bio15, I'll try to give it a go:
On piston engines, water injection was used since the mid 30's(actually, it started with aviation engines and was then accomodated to the high-tuned dragster and competition car engines). With a piston engine, water injection has several effects:
- increasing: the pressure in the cylinder (water damps add their partial pressure to the whole, so without increasing the compression ratio, you get a higher-compressed combustible fuel/air mix)
- cooling effect: Water is injected in liquid condition and evaporate in the cylinder. Since the evaporation is an endergonic process(consumes external energy), it "consumes" part of the heat in the cylinder.
However, running a water-injected recip engine on low/middle rpm's didn't pay off - you have to spray just a small amount of water(or else you'll end up with water condensing in the cylinder) and the benefits are fairly marginal. It pays off only on high-rpm regimes(ie dragsters and other racing cars). As a matter of fact, many of the WWII fighters were equipped with the so-called "emergency combat power unit" which was a water-injection system. The Mustang in particular has this feature.
I'm not that common with jet engines, but I imagine, water injection could help to produce a higher combustion pressure as well, as long as cooling the combustion chamber at the same time. What my sources say, I must agree with JETPILOT - the early 747's had water-injected engines.
Remember the PanAm 747 which struck the approach lights taking off from SFO - this one apparently had water injection and had used it during its ill-fated takeoff to get the extra power needed.