I discovered how to configure a flash drive so they can be used in the USB slot. Off of the root of the drive create a folder named Music. Drag and drop the mp3 files into the new folder.

It has been working for me intermittently, the flash drive I had been using are the typical sized ones, but they seem too long for a good connection as they move a bit as the boat gets bounced around and the music cuts off.

I ordered some mini sized USB flash drives that I tinkered with yesterday, worked great in the driveway. This weekend they will get put to the test on the water.