Good question! As far as I've figured out, there are 3 options, each with its own disadvantage, depending on your make of housing and lens/port configuration. If you have a "proper" wide angle lens like the 7-14, then you can use the technique that thetrickster mentions. The disadvantage is that it's a bit fiddly. You could also use a clip-on filter in front of the dome, like Mangrove's CF-D127: , but this has the disadvantage that at shallow depths (e.g. less than 5m) you'll probably get reflections. Finally, you can use a screw-on filter, which is what I use with my PZ 14-42 lens. The disadvantage here is that it's a flat port, and the so-called wide becomes more like 19mm. Decisions...
I have the GH2 (hacked). I've experimented quite a bit with MWB, and I'd say it's essential to have a good quality red filter (e.g. Aditech/Mangrove, or URPRO). I've had good results as deep as 30m in the Red Sea and tropics. I use a bit of white translucent plastic, like a sort of expodisc, in front of the filter, rather than a WB card.
Hey midael, nice video. That's about the steadiest footage from a GoPro that I've ever seen!
Brought back memories for me, because I was in Palau in Jan 2014... and it rained virtually the whole time. Like when you were there, it was very dark underwater (if you're interested, you can see some footage here that I posted a while back: https://vimeo.com/92704941 ).