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I'm not underwater specialist but I can give you same tips.
- without flash dont go deeper than 6 m. Best colors are at 3-5 meters.
- you can use slow shutter speed, but not less then 1/125 for 35mm glass.
- weight of you house+camera should be balanced for sinking (but just a little)
- you can use scuba diving ballon (at the surface) with 10m rope so you can attach your camera and some equipments to this rope. So if you seen big shark and lose your 5D you can still find it 
- dont change lenses under water
- always make a sealing test before diving

Lucky you!it sounds very exciting leaf,
sorry I can't be much help as  I never worked underwater and am not knowledgeble on this.all I can do is to wish you luck with it I look forward to seeing the photos.Just out of curiosity will you be doing it for fun or you thing it's got selling potential as stock  if it is the latter perhaps you'd be posting them to macros (??)to cover up that expensive production.
both way  wishing you good luck.
oh well may be I will get one of those cheap underwater cameras(vivitar ie)and play with it just for fun:)

I think you have just about everything covered.  Don't forget to plan on something in which to carry it.  I have a Nikon D70 in an Ikelite housing with two strobes, the arms, chargers, etc.  I bought a large Pelican (I don't remember the model) case to carry it all.  The case has wheels and and handle so you can pull it along.  It's safe to check in as baggage on an airplane - just be sure to add a padlock. 

Rene has some excellent tips that you should follow.  The one I would add is to get as close as possible to your subject.  It will make a huge difference in your images.  Also try to take a few and then view them on a computer before taking more.  It's really a lot of trial and error to get it right.  You might consider taking the first few without strobes.  Then add one or both strobes.  This will let you get the feel of handling the setup.  The most frustrating part you'll find is trying to stay still enough to get the shot.  When the subject is moving and you are moving, it can be difficult!  I've not tried to take any snorkeling - I've only been scuba diving.  I have found though that maintaining my buoyancy is my biggest challenge.

Good luck with it.  It's really a lot of fun once you get the hang of it!

Yes, you need the dual sync cord for two strobes.  The one thing I'm sure you're finding is underwater photography is VERY expensive.  lol  If I remember correctly that cord alone was something like $120.  It's a bit crazy.

I have some images on Shutterstock under the name "debljames" and here's a link to Alamy:
http://www.alamy.com/stock-photography/0D35086A-700E-4DD9-9FFB-3DC8A010EB38/Debra+James.html

I don't have the greatest stuff yet as I'm still learning.  It takes a lot of time to hone your underwater skills unless you live in an area where you can get in the water often.  I live in Colorado so I have to wait for vacation each year to practice. 

Postprocessing can also be a challenge as you try to restore lost color and contrast.  It's also time-consuming to identify the various coral and fish in your images for keywording and captioning.  I found this DVD very useful for Photoshop techniques:

http://www.uwphotoshop.com/

Good luck!

one sugestion for underwater photography is make sure you clear any bubles from youre lens once youre in the water I have had a couple of dives where most of the pics were usless as a buble or 2 were stuck on the rige at the ege of the lense this was in my pentax w30 dont know if this will be a problem on youre housing but it is somthing to look out for. the efect is the same as water droplets on youre lens above water unfortunately Ive had to bin (stock wise) plenty of realy nice diving kite surfing and wave surfing pics for these problems

I decided to go the cheaper and someone less robust route.

I got an Ewa-Marine U-A bag for under $300.  I was only snorkeling, so depth wasn't a problem.  Got some good shots of sea turtles and fish in the reefs in Hawai'i.

My complaint about the U-A bag is the maximum filter size of 72mm, while I have mostly 77mm lenses.  I had to borrow my wife's 28-135 to get something to fit.

The new U-B100 was just released which could accomodate the 77mm lens, but wasn't available yet.

Nice thing was it was much easier to pack than a real housing.  Important since I wasn't just on a shooting trip (Honeymoon).

Wanted to share some thoughts.

I have an Aquatica housing for 5d
X2 Ikellite ds 125 strobes
X2 16 inch arms
extension tube
Dome port
canon 16-35L II

I've only done a handful of dives on the reef with it. I was living in the Florida Keys at the time so with all that water around it made sense to try it. Found out real quick that a big rig like that is tough to handle in the current. could not swim back with it against a tough current first time out. Note to self: Make sure to get good fins next time and swim against the current in the beginning of dive

I got the aquatica housing on the recommendation from a pro underwater photographer/teacher in the keys(not a salesman). He told me I had a far less chance of losing my camera due to leakage with the aquatica then with the Ikellite housing. He also mentioned: its not a matter of "if" your housing leaks but a matter of "when" it leaks. That kind of freaked me out a little.

biggest problem I've had is dealing with "backscatter" you have to get your strobes positioned just right as to not light up the out of focus particles in front of your lens. Any type of "on camera" flash would make back scatter a serious problem. Macro stuff is not as bad as wide angle because there are less particles between you and the subject. So far my best selling underwater pics were taken in a swimming pool  fun stuff anyway.. trying to look through the viewfinder is kind of joke if the current is moving. I remember after one dive I looked like I had been in a knife fight, I must have brushed up against the fire coral about a dozen times..painful nasty stuff!! 

Here is few shots I managed to upload if anyone is interested http://www.istockphoto.com/file_search.php?action=file&lightboxID=2605741