Hydraulophone (water-pipe-organ-flute) is fun and easy to play, but after more than 20 years of playing it, I find I can still improve, learn more, and reach better levels of musical expression. Because the sound is produced by the medium that's in direct contact with your fingers (the water), you can exert alot of subtle control over shaping and sculpting the sound. You can play chords on hydraulophone, and you can separately sculpt and shape each note in the chord, and do this continuously while the note is sounding.

Most hydraulphones are acoustic ("aquacoustic") but no matter what kind of hydraulophone it is, the key to good music is to play in the water.