I had weights behind my back initially, but found that moving them to the front helps my dive so I shifted them later.
5. Staying Still/Moving for Shots Will Be Tricky
Some photographers like shooting with a tripod, I like moving around to change my angles and framing organically. This unfortunately doesn't translate well underwater.
It's both difficult to stay still and move in water because, well, it's hard to be still when you're floating, and hard to move/paddle when your hands are occupied with the camera.
The best way I've found for myself is to simply decide a course of movement, go for it, then press the shutter many many times. :D
6. Camera Focusing Issues and Loss of Colors on Model's Skin
This usually happens due to loss of light underwater and being far from the model. Brenda overcomes this by using a 10-17mm on a crop sensor camera so I could move in closer (very close!).
The problem that arises from this is that every little movement distorts and changes the composition drastically. I use the 70-200mm 95% of the time for my work, so it definitely took some getting used to to shoot with a lens so much wider for a complete shoot. I still want to explore using a long lens underwater in the future, hope it's possible. :(
7. Have Extra Hands
Logistics of shooting underwater is painful. Every little adjustment takes a lot longer than it would on the ground. Depending on your light setup, just for clothes/fabrics alone I think you'll need at least 2 assistants underwater.
I only had Brenda so we had one side of the model covered. I ended up using my feet to adjust the fabrics while trying to stay in place for shots sometimes, it's definitely not ideal and more assistants would've helped the shoot move faster.
I also attempted directing+paddling with my left hand, but all I managed was hurt my right pinkie finger for trying to balance the entire weight of the camera and a strobe on it. :(