Bruce....I don't use My Mode underwater. You can't pre-set settings exactly since conditions are always different, they always need adjustments. I'd have to make the same adjustments in My Modes so I skip that step, it's button pushing I can do without!
Mike....here's how I do it....
OK/menu
down arrow to choose WB
down arrow to choose last option (oval with two triangles
underneath)
right arrow to choose it
You'll see a screen with a yellow bar at the top with One touch WB and Cancel and Go at the bottom.
Aim the camera at your slate that's tilted a bit to get full light or at a patch of bright white sand
Hit OK and the camera will calibrate
Hit OK twice more to get out of the menu.
Voila! WB is calibrated for that depth. Remember to do this every time you change depths by 10ft or more or anytime conditions change. Also don't forget to move your WB back to Auto topside, when you use a strobe or you don't want manual WB. If not, all your photos will be too red.