Andy,
If you are referring to Chris's suggestion that fish blood clots quicker under water then there is little evidence to back this claim up.
I was always under the impression that blood coagulation in all animals has little or nothing to do with outside influences and is entirely dependant on internal coagulation mechanisms.
A quick google search came up with this
http://http://www.cpafap.embrapa.br/aquicultura/download/rev_bras_biociencias_revisao.pdf which if you can be bothered to read it, (I only skimmed it), seems to confirm what I have said.
It stands to reason however that keeping an injured fish out of water for any period of time is going to cause it more harm than good so returning it with all haste is obviously the best course of action.