I would suggest looking for a used DSLR set up. A Subal Housing with an older camera can be purchased quite cheap and they produce excellent images. Subal housings are a pleasure to work with and I highly recommend them. D70’s and D200’s with housings are very affordable. D300’s will be cheap soon as people go to the D700.
Manual mode is vital in getting good underwater shots.
With fashion all of you shots will be wide angle and auto modes do not work. Expose the water and use the strobes to light the subject.
I also would recommend larger strobes minimum of 2 strobes for fashion 4 or more would be nice to control the shadows.