Wow, 7+ min breath holds for a newbie, that's quite impressive! I would like to be there
Apart from the psychical aspects, the breath up may have important influence, and cause the different curves of hypoxia and hypercapnia in both cases. In water, the diving reflex should be stronger than when training dry. But since I suspect that you strongly hyperventilate (starting to have contractions that late is quite unusual at a newcomer), these two processes may collide stronger when training in water than when doing it dry, and hence although the contractions may come at the same time, you may be already more hypoxic in water.
