Yeah but I also haven't trained to do underwater scenes for 7 plus minutes at a time between breaths. Like, even for a James Cameron joint I think that's a terrible idea and could actively hurt your lungs.
Not a scientist but I just don't think it's a good idea.
I mean, it's been said that some of the crew of The Abyss legit hated him for making them act underwater, and they felt like they were being put at risk, but I think that it's almost cliche at this point, when Cameron asks you to act underwater, you've either got to commit or go home. He's a diver himself, and I'm sure that he's not asking them to do something he wouldn't do himself. I'm also sure that he has his cast and crew well trained for diving, and there are safety precautions. That person to the left is actually holding an air hose, so she can "ask" for it any time. When I lived in FL as a kid, there was a place that did live Mermaid shows where you watched them swim and do skits through a glass wall, and the cast all had those breathing tubes they would go to after a couple minutes. If those wannabe Mermaid/part-time strippers were fine with an air hose, then so is a class act like Kate Winslet. The fact that she's bragging about her skills doesn't make her sound like she is being held to the part against her will.
I don't think that this is the first movie to have underwater mo-cap. I believe that was for Aqua-man, but I also believe that they are really pushing things farther here, just like James Cameron always does. Contrary to what some believe, a lot of actors want to work with Cameron repeatedly. Sigourney Weaver, Kate Winslet, et all. Zoe Saldana has also mentioned being grateful to come back and work with him again.