I had one of those Poly Planar things fitted and it was useless and after the second trip blew up.
You can play CDs if you have the right player in rough weather. We were playing CD's on the Conwy trip a few weeks ago that you may have read about on here and we certainly weren't stationary or in calm weather and it never missed a beat.
I have one of these on the link below, although I didn't pay anything like this for it, and it has been brilliant. Has a CD player that also plays MP3's on disc, AM/FM radio, Aux input for MP3 players and can power up to 4 speakers without an amplifier loud enough to hear over the outboard.
One of the best bits of kit on the RIB at the moment.