One uses the menu to decide which button starts AF. Possibilities vary with camera model. With Seacam housings one should use the * button to start AF separately from the shutter release. The external housing control for this is right under ones right thumb when the housing is held by the grip. Index finger on the trigger. Very ergonomic.
The EOS-M has a menu item where one decides between AF, MF, and, AF + MF. The EOS-M, though, is pretty weird in the camera control department as there are very few non-menu controls.
Yeah, I'm surprised too that the Nauticam design doesn't allow for this. Curious.
Tom, is not possible to make the Canon camera switch between AF/MF through a setting in the Menu? Forgive the ignorance, I've always been a Nikon kinda guy.