Announce is good idea. Otherwise, most people may think it's some...one asking, if his N900 can act as a sonar. Which may result in whole array of reactions - moving along without reading is just one of them
If it's emitting clicks for sonar, they might not be very musical for a theremin.
A theremin could be fun, though.
Maybe you could use the front camera to calculate distance by the monocular depth cue apparent size of a hand: smaller as it's farther away, larger as it's closer, and whether it's at nearer the left, right, top or bottom.
Interesting, gotta check the accuracy. I wonder if it would more effectively underwater, for obvious reasons (after altering calculations, of course)?
As a rough estimate, the maximum range should increase by factor 9 due to the better sound propagation quality of water compared to air, but the accuracy will drop by about factor 4.3 due to the higher speed of sound (and therefore smaller delay).
The details depend on signal frequency as well as medium temperature and pressure.
I assumed something in the range of 1-2kHz, 25°C and sea level.
