While arcs are cool on their own, I’m interested in using them for hydroacoustics. One of the ways to treat kidney stones uses focused ultrasound to break the stone while inside the patients body, and some of the devices use underwater spark gaps for the wave source. Electrohydrolic forming uses the pressure waves to force metal into molds. I doubt this is possible, but it would be really neat to be able to have shock sintering of materials with this sort of setup.
Lenses for sound work the same way light lenses work, with refraction according to Snells law. Unlike light, where lens materials have slower speed of transmission than air, most lens materials will have faster speeds of sound than water, so the geometry of converging and diverging lenses are reversed. The ratio of reflected to transmitted power is given by the difference in impedances, and shortly, plastics should mostly allow sound to be transmitted, making them useful as lenses, while metals mostly reflect sound, making them suitable for mirrors.