Finally have some REAL scuba photos to show. 
The first one was taken right below the surface so Auto White Balance was more than good enough:

In deeper water, as expected, Auto WB produced overtly-greenish-blue kelps, forever losing pretty much ALL of the kelp's natural yellowish-brown colors so visible to the divers.
The second one was taken about 40 feet (12 meters) down, using Custom White Balance, aimed at a gray tank. I ended up with kelp colors a little too close to the originals; therefore, lost the sense of being underwater - too little blue-green water hue left so everything still looked a little different from what my eyes saw. Since all of the important colors were captured in the JPEG file, restoring the water hue in Photoshop to match the colors I saw was relatively easy to do:

This process also worked very well with the 1080 HD video, producing clips within iMovie's rather limited video adjustment capabilities for matching colors. Overall I'm very pleased with the outcome so the WX5 has officially replaced my SD800 as the main scuba camera.
I have more photos from this trip here. Suggestions are welcome!
I'm still pretty new to the underwater photography / videography and would like to learn more about underwater imaging, using available light as much as possible so that I end up with images and videos that match what I see underwater. Does anyone know of a good UW photographer who uses only available light for his or her "wet" work?