Well, when you are in the water, I know that menstruation slows down and even stops. I myself do not dive when I have my period, not for the reason you just stated, but because my problem is the occasional discomfort from cramps. I don't like to take medications when I go scuba diving so if the cramps are very bad, I just skip the activity altogether.
One thing you should also be aware of is that you might be prone to dehydration when you are menstruating so make sure you drink plenty of fluids if you do decide to dive.
There is an interesting article at Women in Scuba about women's issue in diving. The article was done by Jacalyn Robert of Texas Tech University; Department of Health, Physical Education, and Recreation.
"There is no evidence of increased shark interest in a menstruating female. The hemolytic blood associated with menses may instead act as a shark deterrent (Edmonds, et al., 1992, p. 65)."
And then there's this from a study by Richard Martin:
"It has been demonstrated that sharks are uninterested in menstrual fluids. This is not, as some dive physicians suggest, because the amount of fluid is small and discharged over a number of days. Sharks have an highly developed ability to detect chemicals dissolved in water, down to an infinitestimal 10 trillionth of a mole per litre of seawater for certain amino acids. If even the tiniest quantity of mensus is released into the water during an hour's dive, the incredible acuity of the shark olfactory system may welll be able to detect it. While certain types of blood are well-known to be hightly attractive to sharks, menstrual 'blood' is a complex fluid that is chemically very different from systemic blood. Menstrual fluid does include 'old' (hemolysed) blood, but it has been shown experimentally that sharks are simply not interested in it."