Why would you want to be using continuous auto-focus underwater anyway? Unless you are using a long lens and shooting a person or creature swimming close towards you, it's better to fix AF at the right distance. Normally, with the kind of WA lenses used underwater, the depth of field will cover the subject. With macro, you don't want the lens changing focus point on the subject as that'll make your audience dizzy.
So, unless you have some special reason to be using continuous AF, just lock the focus at the right distance as you start to shoot and you'll have no lens noise.
Because, if the lens is making focus noises that probably means it's hunting and that'll look bad in the image.
I was recently in Vancouver Island shooting Steller sea lions and the AF came in pretty handy. Much of the time I have it disabled but this was a case where the AF worked really well.
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