I would advise you to think twice. I was shooting underwater previously too, and to be frank, you lose all the diving fun. You will probably end up camping at single location shooting a macro to get an ideal shot or lugging your full set of equipments fighting the current.
You would have to pay extra for excess baggage nearly every flight. I would suggest you do videoing. With a bit of WB knowledge, you can get decent videos.
However, if you are keen to get any UW Photography items, I can introduce you to my friend who own such a business, can usually get good price from him.