OK, I'm no expert on either SCUBA diving or underwater photography but I do know a thing or two about general digital photography and post processing so I'll play.
First of all what you need to understand is that, when underwater, the light spectrum of your surroundings look different (duh) and there is no catch all technique to fix this. There are some tools that will go a long way though and red filters and strobes certainly seem to work wonders.
Here's where they fail though. What a red filter does is simply to shift the entire image towards red on the colour spectrum. This is because water acts as a filter that removes lower wavelengths from the light spectrum (red, orange etc). The deeper you are the more water there is between you and the sun and the less light from the lower end of the spectrum reaches you. This problem is solved easily by shifting the entire image towards red but the problem is that the water didn't remove any of the blues or greens and by shifting those colours towards red they become purple, not good. To avoid this, red filters follow a "halfway-there" approach by shifting the entire scene towards red enough for some red to return to the scene but not enough for the blues to really turn purple. Obviously this doesn't fix the problem but does alleviate it to some degree.
It stands to reason that a better sollution would be to improve the quality of the light that reaches the subject (and ultimnately bounces off the subject and into your lens) rather than to try and fix the badly filtered light that reaches your lens. This is a bit like treating the illness as opposed to treating the symptoms. So we get a light source that is much closer to the subject elliminating the effect of all the water which is between you and the sun. The problem with this approach is that it elliminates the effect of all the water above you but not all the water in front of you. When shooting at close range this is no problem as the amount of water between you and the subject is negligible. However, when shooting a subject at a range of say 5m (as you typically would in wide angle shots), it means that there is 5m of water to filter out the low wavelengths from your strobe before it reaches your subject and again 5m of water to filter out the low wavelengths even further as the reflected light travels back towards your lens. The consequence is that objects close by will be properly coloured while objects further away will suffer from the same problem that we started out with.
So what happens when you use both a red filter and a strobe? Let's follow the route of a light beam that starts at the strobe, travels towards the subject and the bounces back towards the lens. For objects close by the light will reach the subject before any significant red-loss can occur then bounces back and reach your lens again before any significant red-loss can occur. So far so good. But now there is a red filter covering the lens which shifts the entire image towards red. This means that the reds will get even redder while the blues will become purple (although not very noticable thanks to the design of the red filter). The result will be an image in which the red colours are much too attenuated.
For objects further away the light will travel from the strobe to the object and undergo the expected loss of reds, hit the subject and bounce back, again undergoing further loss of reds. As it reaches the lens the red filter shifts the entire image towards red in much the same way as it would have done with objects close by under available light, meaning that for objects far away the combination of strobe and red filter will have less of a negative effect than objects close by.
I'd be curious to see how it works in practice and I'll give it a bash at some stage but I think you may get interesting results if you use both but, as you're shifting the light entering the lens towards red, you shift the light emanating from the strobe away from red. In other words, put a red filter on the lens and put a blue filter on the strobe. I'd love to see how that works but I suspect it might be a good way of combining red filters and strobes.
I suppose here is where the real experts can step in and put all of this in perspective from a practical point of view but that is the theory anyway. Hope I didn't make a fool of myself.
