Thanks all for your concern about M9. Actually it’s not as dangerous as your imaging. The height of wave was never over my bottom that day. You can tell from the 3rd and 4th picture in comment. It’s the perspective…The POV magnified the wave as well as your concern about the camera.
I set the focus on 1.2m in advance and waited for coming waves. Lower down M9, keep your eyes on the wave, click shutter then move the camera away along with the moving direction of wave and keep it over head.
Splash was inevitable. Luckily, very tiny…