As advances in technology makes them smaller and lighter, they are becoming more and more popular in swimming pools.
[1] While waterproof Cassette players and CD players were developed and manufactured, their size, coupled with the fact that both had to include an opening mechanism, prevented customers from using them while swimming.
The players became smaller and did not have to be opened, therefore making it much easier to waterproof them.
Typically, waterproof mp3 players have only one exit, intended both for the headphones cord and to be connected to a computer for loading audio.
They are built to be minimal in weight and size, and are rarely capable of playing or displaying anything but audio information.