underwater as in in the sea, or in the swimming pool? lighting will be a major concern, as water filters out red as you go deeper. then there's also particles in the water which may cause "backscatter" and appear like a snowstorm underwater.
not to mention the cost of a housing if you're talking about using a DSLR. my set-up is about $5k without camera and lens..and i think that's considered pretty cheap!
if you need any help, just let me know. i might be interested to assist in the shoot too.