Has anybody tried the Canon 24mm f/2.8 IS lens, or more generally other IS lenses (which Nikon calls VR).
Canon claims that the IS system gives 4-stops worth of image stabilization in terms of shutter speed. If so that would be pretty interesting.
One alternative is to use the Canon 24mm f/1.4 lens, which gives you 2-stops.
Of course f-stop versus IS isn't really a fair test. For moving subjects (for example, fish) using the f/1.4 lens wide open will clearly be better - it would get you 2 stops of shutter speed which might help stop the subject. IS will not help you much with a moving subject.
But IS ought to be a help in a lot of other underwater situations.
If you want depth of field, so you want to be at say f/8 to f/11, then the IS lens could potentially save the day. Topside, one uses a tripod for such shots, but obviously that is very limiting underwater.
Normally the topside rule of thumb is that your shutter speed should be 1/focal length - i.e. 1/24th of a second (round to 1/30) for a 24mm lens. 4 stops of stabilization would imply you could go to 1/2 second.
This review http://www.the-digital-picture.com/Reviews/Canon-EF-24mm-f-2.8-IS-USM-Lens-Review.aspx has photos shot at 0.8 sec handheld that look pretty good. I doubt that this would apply underwater where you float around and have less stability. However it may still help.
I have used the Canon 100mm IS macro underwater, and the IS seems to help in terms of the image that I see in the viewfinder being more steady, but most macro shots use strobes for the light, and the strobe exposure is very short (1/1000 and sometimes much less), so the IS is going to be less important.
With WA shots, there is often a mix of strobe for the foreground and natural light for the background, where IS would be helpful. Wreck diving, silhouettes and other circumstances make IS potentially useful.
Some people use the 24-105mm IS zoom underwater, and might have some perspecitve on how much the IS helps.
I am less familar with Nikon, but their VR lenses ought to be pretty similar.