Water injection on a naturally aspirated diesel wouldn't really do much. However, it would make a difference on a turbodiesel. The only thing it would do is lower the intake air charge temp and thus make it a little denser. Combine this with a tweak to the fuel enrichment, and you'll notice a small gain in power/torque.

You might get away with a very small bump in timing advance, because it's logical to assume that the cooler denser intake air charge will slightly delay the start of combustion.

Also, you'd probably notice a little more consistent performance... your car would not become so sluggish on a really hot day.

The water vapor would also "steam clean" the engine a bit... so if you pulled the head after prolonged use, you'd be treated to an unusually clean set of diesel combustion chambers.

-Joe