I'm not aware of portable steroe that has a recharable battieries in them. They would have to use NiCad batteries to do this... Google shows a "Jeep rechargable boombox" available through Linen N thngs, but Linen things is out of stock....?
I have Bose 251 outdoor speakers in my yard. The sound quality is exceptional, however, in the tub with the jet's on, it's hard to hear the music. (The speakers are off to the side about 30 feet away.) You should have better luck if you mounted the speakers nearer to the tub. YMMV.
Perhaps the Bose Wave radio? It's portable, and has a remote and quality sound. ?