I looked at the vendor page you linked to, as well as pages for several similar transducers. My conclusion is the "waterproof" transducers are just that: immune to water getting inside and ruining the transducer. IMO these "waterproof" transducers are NOT intended for use under water, they are to be used in air.
One big problem that no one has addressed is the huge impedance difference between air and water. A transducer designed to couple energy efficiently to air will not do so when immersed in a fluid, be it water, Fluorinert, mineral oil, gasoline, kerosene, or whatever. To get some idea of the problem, compare waving your hand in air versus waving it underwater (say in a bathtub or sink). The difference in resistance to motion is very large. The air-based transducer is a tiny crystal that vibrates with a very small displacement, but it is just pushing air around. If you try to push water around, the crystal will not be resonant at an ultrasonic frequency anymore. If it were to happen to resonate at some other frequency, how much power would be required to get the same output in water as it produced in air? Immersed in water, there would be some cooling effect, but is it enough to prevent damaging the crystal? I think not. Probably what you need is a magnetostrictive ultrasonic transducer, of a kind similar to what doctors use for ultrasonic scans of human internal tissue.
So go ahead and "play" with the "waterproof" transducer. Solder some insulated wires to it and pot the connections in RTV cement. Wait at least 24 hours for the RTV to set up and out-gas. Then hook it up to the original driver board and see what happens when you dip the transducer in water. An oscilloscope helps to see what is going on. I am not a gambler, but if I were, I would bet it quits oscillating when immersed in water. Please let us know what happens as your mileage (or kilometers) may differ.