Some folks apparently do use them for deco. An audiobook for 90 mins sitting on a log isn't a bad idea, esp. if you have sat 90min on that same a log dozens of times.
I've read several very good reviews about Audioflood's waterproof iPod shuffle. Best price seems to be through Amazon.com. I want one for swimming laps but haven't had the extra cash yet.
I can understand that as, without some form of distraction, sitting/floating anywhere for 90 mins is going to get very boring very quickly. There are only so many times you can practise drills etc. I would imagine that they would tend to keep it to themselves though.
As for boats playing music underwater, I would object to that both from an enjoyment and safety point of view. I want to be able to hear it if someone is banging their tank to attract attention (for whatever reason) or to hear the sounds of ocean life. I can play as much music as I like above the water during the other 99% of my life.