Toss one of these floating wireless speakers into your pool and let your music go global (or at least spherical). According to the manufacturer, the Aqua Sounder ($100 each) has a 150-foot listening range and supports up to 6 (or 10, depending on which page of Grace Digital Audio specs you believe) hours of battery life. You also get a wireless transmitter (which looks like a bulky MP3 player dock) and a user guide. LED "mood lights" (two per speaker) keep the Aqua Sounder visible when the sun goes down.
Alternatively, if you have a pair of speakers lying around and you prefer a do-it-yourself approach, you can make your own floating, waterproof speaker contraption.