Depends what sort of pressure you are looking for, and at what frequency. Sonar systems use them, and so do underwater seismic surveying systems, although I doubt you will be interested in the latter. Cheers
A waterproof loudspeaker won't work, although a moving coil transducer (say a Ling shaker) driving a metal piston might do it. You only need around 20 um displacement to produce a 0.1 GPa plane wave at 500 Hz. A typical sonar transducer won't work as it can't move enough.