I stopped doing it (AF) with the 12-50. Manual focus mode, and I use back-button focus to get close, and use the peaking and repositioning the camera as needed to fine tune.
By auto-focus I didn't mean continuous auto-focus, which hunts horribly (I've also just about given up using it). I mean temporary auto-focus at the beginning of a shot, which I do in manual focus mode by moving the rectangular yellow box to my subject and pressing the "AF/AE Lock" button. I guess I'm using 1-Area auto-focus. Wish I could make the yellow box smaller for more accuracy. Maybe I should be using Pinpoint auto-focus??? But trying it now I can't get the pinpoint near enough to the edges of the display. I badly need to go through the manual again I guess
But even if that temporary auto-focus is more-or-less right, it's often not spot-on and I want to tweak it with a manual focus knob. Sometimes it's difficult to move the camera because of the situation, or you don't want to upset the subject or the seabed by shifting around.
...Even when I use manual focus i focus at the start of the scene and then don't touch it anymore for really small things...
For me also it's primarily for the start of a scene that I want manual focus, but also occasionally readjustments during a shot. I'd expect to trash that part of a shot where I readjust because of wobbles and errors, but not always if I got lucky of if there's important action at that moment.