Here was the trick:
Short answer: I used a picture of sand, from a previous dive, to set a custom white balance.
Long answer: The problem is that MP4 (video) bakes in the white balance. [This is not an issue for stills that I record raw.] So, you must set a reasonable white balance. The GX7 II offers a "underwater" white balance, but as far as I can tell, no one uses it. It also offers a custom white balance, where you take a picture of something white (or grey) underwater and then tell it to use that picture to set the WB. The problem is that for the GX7 II, setting a custom WB takes about 10 steps, too many for me underwater. (I understand the 1st GX7 is one step.) So, I used a picture of sand from a previous dive to set the custom WB before the dive. My other settings are TV with a 1/125 sec shutter. Everything else is auto. (For stills, I also LOVE slow-speed contiguous burst.)