I have used that waterproof transducer and it works ok, but it's blind from 0-25cm(check the specs on ebay). The one I used actually worked to about 20cm. Once your less than 20cm, the reading were erratic and in the range of about 90cm (if I remember correctly). Because sound travels faster in water, the blind range may be 0-30cm. SO, I don't think it will work for you. I have also used some of the HY-SRF05 (SRF05) and the SRF04. Some of the SRF04 were giving me some very erratic readings. My guess was that the mechanical part inside the transducer was loose. I removed the screen from the transducer and then spayed clear enamel on the inside of it till it was well soaked. Sprayed the outside of the board as well. I then let the sensor drain upside down at a 45 degree angle so the excess could run off and not leave a drop hanging from the center piece. Let it dry /baked in an oven at 230F for a couple hours, then cool. It fixed the erratic readings. Why am I telling you this? Well those transducer modules are cheap $2.00usd or less. Make you connections to the board...solder them. Then coat your module with epoxy or polyester resin (2 part). Don't use the quick set type. With the face of the sensor pointing down at a 45 degree angle, heat it with a heat gun/(hair blow dryer). When the resin heats up, it will become thin before it sets up. The excess resin will run off. Let it cure. You can bake it in the oven for an hour or so at about 200F. You now have a sensor module that should work for the range that you want. I'm just guessing it will work. It certainly SOUNDS (pun intended) like a cool experiment. Let us know what happens if you try it.