[purduephotog]

Without a detailed design it's very hard to give ideas or solutions, but some things to think about:

1) Use the panel to charge a battery. Use the battery to power the pump.
2) Use an inverter to drop the voltage correctly.
3) Look at some electronics from flashlights- they have quite a few ways of doing 'constant current' drives which can sometimes be used to do 'constant voltage'. That may very well drive your pump.
4) Rewire your panel to hit your target voltage.

Remember that your panel only puts out that juice under full sunlight- so you really want to be certain how everything is sized.

You don't talk about the height of your waterfall- sizing a pump for the proper head isn't very difficult but if you miss... nothing will work.

Good luck!

(I'd probably go the battery route with a good inverter... but I don't know what you're building- spending 500$ for gear to save 5$ worth of electricity a year isn't worth it).