hahah, you're going to need specialized sound-isolating earbuds to be able to use headphones underwater, which bluetooths typically aren't. There should be clear waterproof ziploc-type bags that are made especially to keep electronics dry in water, but no room to be able to pull a headphone wire through and still have it remain waterproof.
something similar to this
http://www.landfallnavigation.com/aquapaccases.html
if you're going to be swimming in the ocean, seawater is incredibly corrosive and will damage the headphone contacts very fast.
//nice moonzbabysh has a much better site, the waterproof mp3 case looks good as long as you get a extUSB to 3.5 converter