If your spa has a pair of RCA's on the back for AUX input (most Kenwood and JBL marine radios do), you can pick up one of these cables which will not only charge your iPod but give you RCA audio out.
http://cgi.ebay.com/iPod-to-2-rca-cord-cable-audio-sound-car-stereo-charger_W0QQitemZ310081130997QQihZ021QQcategoryZ60208QQssPageNameZWDVWQQrdZ1QQcmdZViewItemWhat I did was buy a clear plastic iPod case with a string / strap and a universal atomotive adjustable phone holder. I put the iPod in the case, then secure it in the cradle. If it should fall out, the plastic cover string will keep it from falling too far (if it is tied off to something with almost no slack) and also protect it pretty well from spraying or splashing water.
If you don't have an aux input, you can still use one of these cables, you will need to run it through an FM modulator with a toggle switch to turn it on.
Since my radio reception is terrible and CD's only hold 16 or so songs, this is a great way to have hours of uninterrupted music.
