Thanks for the info
I looked at getting the upgrade but I'm led to believe can't reset the white balance to auto once you've selected one push white balance.
Yup, you would have to use Manual WB the whole dive. Also you may want to try 'Outdoor' instead of Auto - sometimes the Auto throws things off while Outdoor may be more consistent for post work.
Edit to Add (and just my thoughts)
I have not used an HC-3 Housing from Ikelite, but have used other Ikelite Housings in the past for video and stills.
Of course everyone will have a different take on the matter, but if I had a choice between a Gates and an Ikelite, even if the Gates does not access all the menu items the Ikelite can (especially now that the Gates support the Manual WB) I would go with the Gates due the construction, the design, the controls and the ports themselves. Yes it would be nice to be able to switch back to Auto or Daylight on a dive, but that is not enough for me to think Ikelite over a Gates. If it was a matter of cost since the Ikelites are alot less, then of course that can be the prime consideration. But since you have the Gates already 
If you get a chance to try to test the Ikelite it would be probably a good idea before getting rid of the Gates and going with the Ikelite.