If I understood your most recent question correctly, typically, the closer you are, the better color you'll get. My only guess is that when you had auto WB on, the camera was able to get a point of reference somewhere in the frame? For best results though, manual will usually come out better though.
The GoPro cameras are very good at auto WB underwater for video. They are pretty much designed for it. I'm not so sure about the picture taking quality though. I haven't seen any, so I don't know. IMO, Sealife cameras are better designed to be used with strobes for photography. Without strobes, once below 20-30ft, you need to manually set the WB to get decent results. When taking videos though, the Sealife cameras do add a red filter to the image as well, and I've seen good results with it.
I was on a dive trip Labor Day week in Utila, and the vis was pretty "milky" for me as well. Early September is typically when coral spawning takes place, so that may have been a cause as well. As far as I know though, if the vis is bad, there's not much of a way to improve it in your pics. One other note is that manually setting the WB on the DC1200 can be a bit of a pain, having to go through all the menus to get to it... Especially if you need to do it somewhat quickly. The DC1400 does have a "hot key" for setting it though, which makes it much easier.
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I forgot to mention, a few of my Utila pics are posted a few threads down. If you look at the pics of the spotted eagle ray, you can tell that I had the same issues with the "milky" waters as you did.