This sounds a lot like the debate with grizzlies- and they don't seem to preferentially attack menstruating women, either. I suspect it's just spread around by a few manly men who would like to keep us little women home and safe.
But, anatomically speaking, I doubt very much that menstrual blood gets in the water if a female is wearing a fresh, properly-sized tampon. The vagina is watertight (unless the water is forced in, as in water skiing- ick). Water doesn't get up there to wash the blood out. That is why tampons don't expand when the wearer is in the bathtub or under the sea.
Also, for convenience's sake, if you are taking the Pill, ask your gynecologist how to manipulate the placebo week to avoid having your period when you plan to dive. (Unless you dive all the time, in which case you have no reason to whine, you lucky devil).