First shooting over/unders takes some prep. Of course very wide or Fisheye lens is needed. If I am going to use strobes I set them up with very long arms, 18-22" per strobe, so they can hang underwater to get light on the coral head, fish, whatever. I shoot on the vertical. The bigger the dome the better. I will often first scout out an area looking for something of interest underwater. If shooting in the shallows, where there is a sandy bottom. I may leave the fins on the boat so I can fill my BC up and use one toe in the sand to balance. Expose as much as possible for the air half, easier to do if you use strobes underwater.
For "Open Water" a little prep with weights. I have a back inflate BC so I try to shift my lead closer to my tank so I can float more upright. Fill the BC up as much as possible. If not using strobes I have tried adding a few Stix arms to the housing just to get it to float a little more.
Not great examples, but I do like trying

