In my experience you need a red filter firstly, followed by the manual white balance.
UThe camera will "see" that they are missing from the spectrum and add them up to a certain degree.
Ive tried the red filter ages ago but it was so long ago now I cant remember if I MWB with it on and if I did it would have been to a white slate. I'll give that another go.
As for MWB up the light column.. Here in Sydney I'm lucky to ever see the light column under 5m. Its green water here.. Boo ho
Adding the red up to a certian degree is the problem I think the camera has. As Peter said MWB on a mid-brown coral/color seems to yeild better results than your standard white. As it seems to give the camera a redder base point to start from with its calulations on WB.
Drew, Yeah dude read them all and tried pretty much everything thats been suggested on here.
Alas still no luck.