It is quite difficult.
I find that it's particularly hard to get proper exposure, definately need to shoot manual, and try many settings, checking your histogram after each shot.
Calm water helps alot, particularly if you can stand up in it.
I try to find something close to me underwater that I can light with my strobes to get some color underwater.
Always shoot with the sun, I find early morning or evening is easier to balance the light above and below the water.
Of course, lenses with diopters won't work, (can't focus above water)
I think fisheye lenses works best
I have heard of people using a split neutral density filter to help balance the exposure, might help.
The bigger the dome port the better, I use the 9" sea & sea port.
I don't think that I'm very good at this sort of shot, but have seen examples that make me want to master the technique.
Take care,
John