A killer music library is no good for an outdoor party without the proper speakers. This seaworthy gadget should do the trick. A small transmitter hooks up to your sound system (which can be your iPod, phone, or anything else with Bluetooth capability, as well as your computer), and transmits music wirelessly to the speaker. The speaker is submersible in up to 3 feet of water and takes AA batteries.
About $40, Nextar