I took Cam's advice. I bought a car radio that has one DVD/CD-R/CD slot plus a slot for a memory card and a USB port to plug in an iPod-like device.
I paid $30 for a program that enabled me to change all my CDs and old cassette tapes into MPEGs. Instead of carrying a locker full of music, I now have to remember where I left the remaining few CD-Rs.
Then there's the cockpit speaker issue. I found I could drive radio linked speakers, which are AA battery powered and work anywhere on the boat, dock, beach... So I did not need to drill holes and feed wires or worry about water proofing (I don't listen to music in a blow).
Since I have a lot of AA battery charging options and a ton of AA batteries on board anyway, this seemed a good way to go.