I shoot in manual unless I know I need to follow fast action where the subject might be below me one second and above me the next (exposure can vary quite a bit when shooting down vs up in the water column). Like others, I usually lock down my aperture and shutter speed and then adjust exposure using ISO only, or go into AutoISO where necessary.
I like knowing what my ISO is at though, which is why I shoot in manual. It lets me know the conditions in which my lights are going to perform well and when they're going to be too weak to bring back colors. And with the excellent layout of controls on the Nauticam GH5 housing, it's quite easy to adjust the ISO on the fly manually.
In terms of monitoring, I primarily rely on zebras to tell me what's blowing out and the camera meter -- I find that -2/3 to -2 stops underexposed for ambient light (according to the internal light meter) usually produces the most pleasing "blue" water column color for me. So that's what I aim for.
For those of you who do use the waveform, what IRE do you aim to expose the background water column at, generally? I'm quite curious what IRE level of blue people find most pleasing as a backdrop.
And by the same token, what do you set your exposure by? Topside, I usually expose to get the skintones of people in the right IRE range, and let the rest of the image fall where it may. Since underwater people shots are a lot less common, I'm curious what everyone sets their exposure by? 18% grey? The water column? Expose to the right just shy of blowing the highlights and fix it later in post?