When I'm working underwater I prefer to leave the scuba out when possible and to breath-hold instead. That way the models and I need to come to the surface to breathe between takes and I can give them instructions for the next dip down. Scuba can slow everything down, just because the kit is cumbersome. Here in the U.K., the law requires a minimum of three commercial divers per dive, and that can make it too expensive for some budgets.
There's a tank in north London that is 10 metres square and can be any depth up to 4 metres. There are windows in the side, but I don't like using the windows. It limits you so much to a narrow view of the subject, and it is harder to communicate with your models. I've seen so many good ideas fail because someone tried to shoot that way. I always like to be in the water with the subject.
We don’t tend to rehearse so much as just shoot tons. The more you shoot, the luckier you get.
I also like to breath-hold for young children. It's scary enough with my big black mask and a big black cap. If I arrived in full dive kit, fins, wet suit, and everything else I would probably make them all cry immediately.