Normal speakers don't fare all that well at the bottom of a swimming pool, so it should come as no surprise to learn that underwater speakers come specially designed to not only deliver quality audio in a submerged environment, but also to avoid shorting out, electrocuting swimmers or turning to driftwood.
It's a rather niche corner of the audio market, so you can expect to drop anywhere between $300 and $2,000 on a quality underwater speaker. Typical models consist of a watertight, waterproof plastic speaker connected by a length of insulated speaker wire to an above-water sound system. You might choose to install the speaker inside an alcove in the pool -- in which case you might use an acoustic sound lens to direct the sound waves out into the pool -- or you might just attach the speaker to a rope and dangle it into the water.
A good underwater speaker won't sound all that great if you use it out in the air. In fact, it might sound downright awful. According to Clark Synthesis President Bill Phillips, a quality underwater speaker is more like a fine wooden instrument such as a cello or violin. The sound radiates off the entire surface area of the speaker rather than from a focused point.
The volume of water you can cover with a speaker varies. For instance, Clark Synthesis recommends one of its Diluvio pool speakers for every 20-foot-by-20-foot (6-meter-by-6-meter) section of the pool. The Lubell System 3300 Diver Recall System -- intended for use on dive boats -- can sound a siren heard across an entire 1-mile (1.6-kilometer) underwater radius, assuming optimal conditions. Dive speakers such as these enable instructors to communicate with other divers, but they can also blast funky beats for man and fish alike.