You know, there are only a few activities that I routinely say to avoid in pregnancy -- horseback riding and scuba diving. Not that this is based on any specific research, but having participated in both of these, I understand their joys but also their dangers.

Having said that, if you are a very experienced diver and you don't go down far enough to experience the risk of nitrogen excess, I suppose that a recreational dive is not going to hurt. But if I were you, I would stick to snorkeling.