Your lighting and subject matter are very effective. Her facial expression is very compelling, but as far as the positioning goes it doesn't look like it would be as comfortable or relaxed as her face may suggest. The curve of her rear is beautifully accentuated by it, but the twist of her spine and shoulders looks less inviting and more like an awkward in-between-switching-positions. Perhaps if the model's upper body was moved slightly to the left, raising her left shoulder slightly, we would get a more relaxed looking position and a better view of her beautiful figure. Moving the frame itself slightly to the right of where it is with this changed position may allow for a reflected shape between her back and the bathtub, accentuating the curves and positions of both her upper and lower body visible.