What exactly do you mean by "set manually the white balance"? The first 30 seconds of that video is very very blue. The final 30 seconds has the nicest balance (to my eyes).
On Canon 7D you first take a picture of something white in RAW format, I use a white writing slate.
You then select from the menu "Custom WB" and select that picture, press SET. Now you have a custom White Balance setting that should work for the situation where you took that reference picture.
Then press WB, and select custom. The more "usual" setting is WB = Auto.
But custom-WB isn't going to do miracles for anything shot at great depths with only available light.
On that video the first 30 seconds are from the deep and early in the morning, where there the very strong rising sun light above the Red Sea has been filtered and only blue remains, there is a trace of red there, but it is so weak that even setting custom WB in attempt to compensate that, doesn't help much for video. If you look at color histograms from RAW images taken from that depth, you can see that the blue light is almost 1000x stronger than red. At noon you get more red color to the deep, and the red-blue balance gets better, maybe around 1:200.
The latter parts of the video are from 15, 10 and 5 meters, where the red light is so much stronger that setting custom WB starts to work really well. In fact you might get too red images, if you don't set the WB frequently enough, as the intensity of red changes rapidly on each meter of depth.
If you take still images in RAW, you can get much better results from available light shots at post-processing than shooting JPG (even with custom-WB), and you get better results than with video as well.
For RAW images, setting custom-WB has no effect as such, since you balance the colors in post-processing. Canon 7D CR2 file stores the full 14 bits (4096 values of intensity) from the image sensor, and the color transformation from the Bayer mosaic is calculated in post-processing. A JPG and video file only stores 8-bit (256 values) data, where the color transformation has been calculated on-the-fly by the camera based on the WB setting (on in WB=Auto mode based on what the camera's WB algorithm has calculated).
