It should have a powerful Bluetooth compatibility feature to connect with the earphones 3 meters deep in the water without any signal distortion. It must have a shuffle and play-to-play option to allow you to switch or select your songs while swimming. Battery life must be 6-12 hours, but it varies because of the different models. But, It is highly recommended for you to choose one which may work at least 7 hours after every charge.
The majority of the MP3 players supported different file formats, including MP3, APE, FLAC, WMA, WAV, ACELP, and AAC-LC. But some of them only support MP3 and WMAs. We recommend you buy the one that supports multiple audio formats. It should allow you to download music from different music streaming services and enable you to transfer them without changing the file type.