George, if I may just add my 2 cents:
First, Caribbean waters can be a real bitch if it comes to white balance due to their very intense blue color. I remember having some issues as well when diving Turks & Caicos 2 years ago - and I wasnt the only one.
Second, I would tend to agree to your theory, that the yellow shool of fish was causing some offsetting adjustment in auto-mode, resulting in that magenta cast. I am not convinced a meter or 2 difference in depth during the clip on its own is causing such a heavy shift, but it may be a contributing factor.
Third, I assume this was shot in ambient light without a red filter? I would suggest using at least a red-filter. Also well worth trying out different ones. I have used Magic Filters as well but I like the results I'm getting from URPRO filters more - maybe a matter of taste. In fact, these day I am not using a red filter but a magenta filter which on the AX100 in AWB gives a better color balance (details are outlined in that article by Pete Lightowler that someone already here posted before).
Now you may say: I already got too much magenta in my picture, how can a magenta filter help? Scientifically maybe correct, but this is not really about science but trial-and-error ;-)
And of course a good set of video lights almost always helps. Not on the widest wide angle shots but on most else.
NB: that turtle really looks quite interested in and approving of your equipment :-)
good luck!
