This is when shooting without strobes or with strobes?
At the risk of repeating myself....
Without strobes you should manually WB.
With strobes you should Auto WB.
With photography 'rules', you should know that they can be broken if you want.
1. No Strobe.
Manually WB roughly every 10' or when lighting changes significantly. I use the palm of my hand. It gives me a realistic review, but can be changed later in editing.
2. With Strobe.
Auto WB. Understand the limitations of strobe strength. I recently bought a Inon S2000 which (like most strobes) has a working range of about 3'. Anything beyond that distance will not receive light from the strobe, so you're back to ambient light. Foreground should be realistically lit but the background will remain blue. If you intentionally shoot looking away from the reef then this is not a bad thing. If you're forced to shoot against the reef, then a blue/cyan reef can look washed out, and lacking in vibrance.
You can artificially increase reds sometimes with powerful applications like Photoshop's Channel Mixer. I find that at depths beyond 3ata it is difficult to get accurate pics with good colour. Most colourful shots are taken in the shallows (<2ata), and often the shots obey the simple rules:
1. Get close, then get closer.
2. Get the sun to your back or over one shoulder.
3. Shoot at eye-level or from underneath.
Depth: 1.3ata, Distance: 3'. Ambient light travels 13' through water to reach the sensor, strobe light illuminates foreground objects.
f8, 1/250, ISO 100, Auto WB, single INON S2000 strobe.
Compare with:
Depth: 3.5ata, Distance: 12'. Ambient light travels through 95' of water.
f2, 1/125, ISO 160, Manual WB, no strobe.
Manual WB helped me 'find' the Napolean while reviewing the shot. If I had a strobe at that point, I would have been able to put colour in to the Napolean. The background fishes would be unaffected by the strobe as they are too far away.
Finally:
Depth 4ata (I was at about 3ata trying to fit as much of the school as possible. Distance: approx 60'. Ambient light travelling through 150' of water.
f2, 1/125, Manual WB, ISO 125, no strobe.
I doubt manual WB did anything to help this shot at all- there is no colour at all. No strobe in the world could light this shot. Distance is far too great.
All shots are edited in Lightroom and/or Photoshop. Now that I know more about the 'channel mixer' I may try to adjust the deeper shots; maybe I can improve them.... maybe not.
The critical element of all the shots is the amount of water that light has to travel through before reaching the camera sensor. This amount is your depth + distance from the subject.
Manual WB can definitely help your shots without strobe. With strobe, just leave it on Auto and concentrate on other things.
edit:
I fooled around on Photoshop using level adjustments for the deeper shot.
