The video below is a demonstration of the Minamo interface (Minamo means 'surface of water' in Japanese). Creator Taichi Inoue writes that he made Minamo because, "Water is the most intimate material for [the] human body." The process involved placing a tank of water over an LCD display and using a webcam and a light to detect when the users finger breaks the surface. Taichi says a "color tracking system on Max/MSP+Jitter finds the coordinates where the finger appears" and moves the cursor to the spot.
Taichi doesn't provide much else in the way of details but it's an impressive device, even if we can't really see any use for it. Don't we normally like to keep big tanks of water away from our gadgets?