Have a read at this:
The paragraph that may apply to you is:
"If you need to use AF while shooting video, the best method is to turn Continuous AF off (this is important) in the menu, set the camera for AF and touch the screen whenever you want to refocus. Half-press shutter also works in this mode while shooting."
I shoot with Panasonic LX100 and this work, I use "back button focus" before recording, then start recording, if the subject moved, I can press (and hold) the shutter button halfway to refocus, sort of create a slow focus pulling. This work for fairly slow subject. For fast moving subject, I think AFC with Continuous AF on is better as long as you are close enough and the water is relative clear to have good contrast between your subject and other stuffs in the water.