The most boring part about swimming is that you don't get any sensory stimulation beyond staring at the bottom of the pool. And given that electronics don't generally play well with water, there hasn't been much in the way of portable music that works for a swim workout. Finis is making one of the few units designed for underwater use, the Swimp3. Rather than use headphones, which can easily slip out of your ears, or fill up with water, the Swimp3 uses bone conduction plates to deliver sound without needing air. Check out the video review for more.
The unit doesn't use earbuds, but rather bone conduction plates that deliver sound by vibrating your skull instead of the air (since there isn't any underwater, duh).