Yup. A person can always go through the menu presses by looking through the housing, aligning the dial to 'one push' and then aim at whatever. However I don't like taking my eyes off the action, I have missed too many things that way. After now using the housing on several trips I use two methods:
OK. I used the camera and housing in the Red Sea for the last 2 weeks. I didn't find MWB too hard. setting up the menu to get to the one push button required tipping the camera sideways so I could glance through the window on the side to see where the pointer was pressing.
Once I got to the one push I could hold the camera in the left hand (holding the left hand grip) and use my left thumb to push the button while my right hand held a white balance card (or I used the palm of my hand)
The biggest problem was then finding the OK button to leave the menu. I got very frustrated trying to do this and continually hitting the one push button again.
In the end I resorted to using the ON button on the back of the housing to cycle through from camera to memory to playback and back to camera.
This kept the white balance setting and, while taking a few seconds to do was more reliable than making multiple attempts to hit the OK button.
I made a few experimental shots comparing AWB and MWB and the colours were much better with MWB