Pink tinges: I'll bet the problem is due to the fact that you've got multiple light sources (flash and overhead), and they are being filtered differently, and different parts of your photo are getting different proportions of the two light sources.
For instance, a foreground subject lit mainly by flash, much like your slate, will give bad background colors because the background color is set by the ambient light, which has been filtered by a whole lot more water.
You'll see this a lot in above-water photography too; frequently people's faces are much whiter (bluer) than the background when fill-in flash is used. (Not just exposure; even when the exposure is fine the color temperature is different.)
Solution: the only solution after the fact is to retouch, I guess. Since it's hard to "pose" your subjects underwater, there's probably not much you other than this.