I use a Stix float belt and six Stix jumbo floats on my ULCS arms when shooting macro with my Z240s and the rig is slightly negatively buoyant. When shooting wide angle with an 8" dome I only use the Stix floats on my arms.
This is how I do it with both my 7D and 7D MkII in Nauticam housings:
I setup one of the custom mode settings (there are three on the 7Ds mode dial but only one on the 70D) so that Metering Start only is assigned to shutter release at half press. When I want to shoot with manual focus I switch to this mode so that I can focus manually and when I press the shutter release AF will not occur. Note that you can always do auto focus in this mode by using the focus now button on the back of the camera. When I want to go back to normal shooting I just turn the mode dial back to the setting I normally use (Manual) so that AF is once again possible with a half press of the shutter release.