The GSI Outdoor Wireless Underwater Floating Speaker is a full-range three-watt speaker. Just plug your audio device into its wireless transmitter and it will pick up the signal from up to 100 feet away. Not only will the speaker float on the water's surface, it's rated to submerge up to three feet (IPX7 grade waterproof) and keep on rocking for 30 minutes. It also has an internal LED bulb, so you won't lose it in your pool at night. It does require 4 AAA batteries (not included), but you get a case to keep it in.
List price is $89.99 US, but you can get one for just $69.99.
Source: 7 Gadgets