Water organ as known as hydraulic organ, which is invented in Greek from 3rd century BC.
The first water organ is invented by a scientist called Ctesibius of Alexandria.

Basic mechanism of water organ:

The mechanism is operated by converting the dynamic energy of water into air pressure to drive the pipes. Hence its name hydraulis, literally “water (driven) pipe (instrument).” And until now, we only can found the ancient water organ in Dion, an ancient Macedonian city near Mount Olympus, Greece.

Nowadays, there is hydraulophone which is invented by Steve Mann. And the largest hydraulophone is in Ontario Science Centre, Toronto. However, it is in diatonic scale, which is difference from original water organ. And the mechanism is far difference as well.

By Glogger – Own work, GFDL, Link