I put this post in here because people ask so much about this. I totally understand the urge to know but please note that this just the way I set it up. There is so many other ways to do this with wonderful results.
So, on the camera side I go with two Canon 5D’s and there is a reason for two housings. When I put one camera inside my UV housing I need a secondary camera to shoot topside – and if – god forbid – the underwater housing should leak, I cannot shoot more images this trip. So it adds both flexibility and safety – and additional expense of course. The underwater housing is aluminum and very robust. It is also light weight and as small as possible for easier travelling. The dome and flat ports is made of acrylic. This means it scratches easier than glass, but at the same time it is easier to fix. I bring small bottles of acrylic repair kits that I can rub clean if it scratches. Works wonderful.
Lenses is a wonderful world of opportunities the way I see it. When packing a telephoto lens or a 15mm wide, I can feel it tickles in my explorer nerves. I have a wide variety of lenses and seldom bring too many when travelling. I try to cover the range needed with around 4. Starting with 100 macro, 24-70 workhorse, 70-300 favorite and 15 or 14mm for mainly underwater work. Often I also bring my 50, 1.2 just because it is absolutely wonderful.
On special trips where I need more creative freedom I sometimes bring 45 tilt/shift that has some magic posabilities.
Then I pack a variety of lights. Some solar 1200 lumen underwater lights, 3-4 underwater flashes that runs both on cable and remote from Ikelite. And I usually bring 2 remote SB flashes as well. ..so by now, I only need two t-shirts and a swimming trunk to exeed the max allowed weight for air travel but my excitement is very high. That means that my tripod usually is small and lightweight. This is not ideal but with a cable releaser for the camera it does the trick of keeping the camera completely still.
The modern memory cards is very robust and can actually drown and still hold data. I have accidentally washed a 64gb flashcard in a washer and dryer and it still works perfectly. It is not the recommended way to clean images but it is interesting to know. I travel with lots of cards and I run backups as often as possible over to my computer. I never delete anything on the road and I would probably get a mental diagnose for running backups. Whenever I have internet I also do dropbox just in case everything was stolen or damaged. The images is the only thing impossible to replace.
Diving gear – I usually only bring my long time favorite mask and off course a small bottle of Johnson & Johnson baby shampoo. It is just the best antifog product ever produced. Who found that out?