I use a micro SD card in my phone and have saved all my music to that. I'm amazed that I have a gazzillion MB of music on that small chip that is 100 times the music than the briefcase I used to carry around with all my cassetes in the car .....
I have a USB stereo, Sony from Crutchfiled, and connect via the phone. I can control music and volume anywhere on the boat. Also have a Bluetooth speaker that I use while sitting at the bow or when we go to shore. Just make sure you get one that floats ( ask me why this is a good suggestion
) This saves streaming data or connection issues on the phone.