We went through this discussion as well and have decided to go with a combination of outdoor speakers/ rock speakers and the inside stereo. I have a 200 and 300 cd jukebox style CD changer, so this provides the maximum flexibility.
I think that the stereo would not be appropriate for the listener in the spa, just because of closeness to ears.
I would recommend the Polk Atrium series of outdoor speakers. Polk is has always been a bit better value than Bose, and the listening test side by side with the Bose 151 suggested that theses were an appropriate buy. The series 45 at aobut 150 per pair is a good dollar/value. There was not a big enough difference to merit the upgrade to the 55 series.
For Hot tub use, I don't think that it is necessary to go high end in terms of ouside speakers just because you are sitting in the water and it is a bit noisy anyway.
Klipsch makes a nice outdoor speaker, but at $500 a pair it was too pricey.
I am going to try a set of those rock speakers. First, I am going for a set of Sonance hometech at best buy ($80). Had not had the opportunity to hear them yet, but is was better than the $500 for the Rockustics or Niles Audio model.
You can find outdoor subwoofers as well.
