If you look at issue 274 of the Australian Silicon Chip magazine, there is a project for an ultrasonic antifouling device for boats.
I have built one of these, and the transducer is actually resonant at 40 Khz, but they pulse frequencies of between 19 and 45 Khz into it.
I have no idea if it works, but as it gets near the resonant frequency there is a noise and ripples on the water surface ( I had the transducer against the side of a bowl of water )