The Neptune speakers rest on your cheek and make the bone vibrate so that the vibrations can be relayed to your cochlea, allowing you to hear music. The process is called "bone conduction," and is also used by some marine mammals. Since the music is going straight into your head, you don't have to deal with anything in your ears while you're trying to swim.
The Neptune is an updated version of FINIS's years-old SwiMP3, which had 128MB of memory and sold for $180. Now at 4GB, the Neptune costs $160. A quick PSA, though: Just because you feel the music in your bones, does not mean you should engage in any type of swim-singing or swim-dancing unless you are totally sure no one is watching. More here.