During Al Gore's Earth-a-polusa or Earth Aid or whatever it was called, XM aired a nature channel for 3 days. It was a cool channel although the playlist was a bit shallow. I heard the same ocean several times over those 3 days.
Slacker has a lot of nature tracks, but without knowing the artist I've only come across them by listening to their New Age channel. I've slowly added them to my meditation spa channel, which still isn't where I'd like it to be, but getting there overtime. One day I am going to end up with a really kick ass meditation channel. It works very well now for going to sleep, but now and again I'll get something pop up off the wall that I have to ban that isn't right for the channel.
Over the last few months I've checked out nature sound channels all across the internet radio world. Most of them are horrible, loud drums and chanting, the stuff drives me crazy, doesn't make me relax. However, I have found a few artist here and there. Pandora has a pretty decent spa channel, but since it works with sending only a couple of artist, you really have to listen to it to find out who the artist are they they will play. Not ever artist played on Pandora is available on Slacker either, but mainly because Slacker doesn't add the various artist type music like, Piano Relaxation (light and tender piano tunes) or Stream Relaxation (water streaming down a brook or over a water fall, very nice, occasionally a rainstorm with some thunder off in the distance), which is nothing more than a bunch of various artist, but it is good music.