Whats wrong with headphones?
Buy a couple of cheap sets with a double jack and waterproof your mp3 player in a few balloons with zip ties and silcon and your done. make sure the wire has a socket for the headphones to pull in case of emergencies. Maybe a small bungee also to pull the excess wire in.

Alternativly you could also buy self amplified speakers for your mp3 with D size batteries that could be waterproofed and mounted.