The camera loses focus when filming

[Taylor815]

I'm filming with a Seafrog underwater housing and a Sony A7Riii.
The camera seems to "lose focus" from time to time while filming.
Check out the link below from my Youtube channel for an example of me filming a Yellow Boxfish.


I assume that my focus settings are wrong. But what should I set them to?
The lens is a Sony-Zeiss 16-35/4

Please check out my other underwater videos from the same channel.
Click on my name under the movie box and you will be able to see all my videos there.

Thanks in advance