The simplest depth sensor is using a tube sealed on one end, you connect an air pressure source ( hand pump etc... ) to the tube. Put air into the tube so that it just starts to bubble out the bottom of the tube, the air pressure required to displace this water will be the same pressure exerted by the height of water column displaced... Convert to height, calculate volume, the Freescale mpx10pd would measure approx 5ft of water depth.
Sorry it's not ultrasonic but this style of depth measurement is very common in industry!