Underwater in Belize
So here are some details on the underwater shots. Keep in mind I had no idea what I was doing going in, and still don't. I used a 'housing' from Ewa-Marine. It's actually more of a fancy plastic bag. It did the most important part of its job very well, my camera never got wet. I was using a D7000 with a Nikon 17-55mm f2.8 and an SB-600 speedlight. I also had an additional 3kg weight in the bag. The whole thing weighed a ton out of water, but still floated.
Controlling the camera was tough. A lot of reviews said you can't change settings easily, so shoot in auto or program. I found that you can change modes pretty easily, and even aperture while in the bag. My main trouble was keeping the lens at the focal length I wanted. The bag tends to compress the lens down to its shortest physical length which is 38mm. I really wanted to shoot at 17mm for those half water half air shots. That and the flat front element of the bag probably made it hard to get the effect I was going for.
I ended up shooting in aperture priority after playing around with settings. Looking back at my shots I probably should have used a larger aperture (f4), but focusing underwater is hard, so I was looking for more depth of field (f5.6-8). It's really hard to look through your mask, the bag, and then the viewfinder. What I did find helpful was back button focus. It was hard to feel the shutter button, so being able to use a button on the back of the camera to set focus without triggering the shutter was helpful.
A lot of the snorkeling was pretty shallow water (3-8ft), so there tended to be pretty good light. I only used the flash for under the pier. I found the best way was to shoot all manual. I dialed in some basic settings f8, 1/250, ISO 400, and found a manual flash level that gave good results (usually 1/8-1/16). I ruined some potentially great green turtle shots on the first day thinking that I needed flash. They all came out overexposed because I was in aperture priority with the flash on TTL.
In the end I had to do a lot of white balance correction in post, and I had a lot of out of focus shots and motion blur. It isn't easy to get sharp shots with sediment in the water, plus both the camera and subject are usually moving.