He he, I was just talking to Alex recently about "reciprocal" filtering on the strobes. He put me straight, reminding me that the strobe filtering is totally dependant upon the water filtration of the ambient light, i.e. the filtration on the strobes would ideally have to vary with depth - not an easy thing to do.
The reason for this is that the lens filtration (red, magenta or magic) just balances the RGB channels in a gross fashion, so that the red channel in particular doesn't bottom out. Then the WB does the fine tuning of the colour balance. If the depth was such that the filter was slightly too strong, then the WB will actually turn down the red to give good balance. If in this scenario you'd filtered the strobes with the reciprocal cyan filter, it would give a noticable cyan hue to anything hit by flash, due to the WB turning down the red channel.
The most extreme example of this would be using the rig above water.
Basically for any strobe "gelling" to work in any scenario, the strobe filtration has to match the ambient colouration, hence any strobe filter you pick will only work for a certain range of depths in the water column. I'm guessing that's what Robert has found in his shots. Oh yes, and that extra filtration factor between the strobe and the subject will mess things up too. 
Maybe if we had a ringbound stack of filters on the strobe and could just flick over the right number of sheets for each depth... getting ugly isn't it? 
Martyn
