One other thing. The challenge I gave myself was shooting a manta and getting the top/bottom lighitng balanced on a frontal shot. Near the surface, the sunlight will dapple their back beautifully, but the underbelly can be dark so I spent some time trying to get my strobes to fire up from beneath the dome at just the right power. I did this with white pointers in South Australia.
You will have mantas for hours, so you can get plenty of practice. Once I got the lighting balanced, I never got one of the animals to swim directly at me because they start to ascend from a few meteres out to fly over your head. The closer to the surface you are, the further out they start their ascent glide path.