I had a go a few years back at shooting ballet underwater having got the loan of a private indoor swimming pool which was about 2m deep.
I thought long and hard about having a scuba-rigged assistant who could feed air to the models. But the models had no scuba training and I had slight fears about breath holding etc. I just didn't think it worth the risk. In practice I found that the time they could hold their breath and pose was reasonably good.
The biggest problems I found were ones I had not thought about: the micro-bubbles generated by a swimming pool pump and the way they adhered to the domeport. The vast majority of the 300+ images I took over about 90 minutes were ruined. To say I was gutted after spending weeks setting the shoot up was a minor understatement. And then I hadn't thought about the deterioration in the visibility caused by all the movement.
One issue I didn't have to resolve was securing safely overhead, out-of-water lights. I used strobes in-water.
The models had ballet experience and, in general, were pretty good underwater. it was noticeable in post however that underwater modelling requires an even more eagle-eye to physique than above water.
It was a fascinating experience that the whole team really enjoyed and the models did end up with a few good pics for their portfolios. The learning curve was massive. I came away with a number of thoughts for next time: get a warm pool(!); perhaps try and find models who had some experience for synchronised swimming; keep checking the domeport (!); and check out overhead lighting.