Grace Digital Audio Aqua Sounder
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
(US$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