Overall I think you did a wonderful work on the photography. The lighting is great, really offsetting her skin and making her appear to glow. I also appreciate the fact that you didn't pose her in something more provocative, or nothing at all. I know plenty of guys might disagree with me, but sometimes it is best not to see everything to make an impression. For that, I'd like to give you 4 and a half stars on your technique.
Originality I'm giving 3 and a half stars based on the fact that, well, girls in bathtubs isn't out of the ordinary. However, I'll give the caveat that I prefer it. Something doesn't need to be original to be beautiful. Vision I'll give 4 stars, primarily because I believe (subjectively) that you had one in your mind, and executed this shot with minimal background clutter.
The overall impact I'm giving 4 and a half stars due to the fact that combined with your own photography work, the model you chose is stunning. Very classic face, high cheek bones, amber-hued brown eyes and brown hair. Perhaps you could have gone with leaving the hair less wet, but then again that is me being subjective on the matter.
Overall, a very exquisite woman with an excellent photographer's eye makes for a wonderful shot. I would give your 4 to 4 1/2 stars overall.
The picture balance is just okay but I'll get to that in a few sentences. The lighting is nicely balanced and even. I really like that there is no distraction from the model (as in other distracting colors in the shot). The tones are excellent. Something about the crop I'm not crazy about. I might have moved this shot to the left and up about 6-8 inches so her whole head was in the shot. But I like the aspect ratio of the pic. You could crop out the knee and it would still be wonderful. I don't like the grey pipe in the shot and would have probably ps'd it. You asked for criticism on this so I gave it a shot...In all honesty I probably couldn't compose half as good a shot under the circumstances. Cheers.