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 [[xref:http://www.gracedigitalaudio.com/aqua-sounder-floating-wireless-speaker-p-17.html|Aqua Sounder|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 [[xref:http://www.instructables.com/id/Waterproof-Speakers-that-float-quotIt-floats-/|floating, waterproof speaker contraption|floating, waterproof speaker contraption]]