How come when we were younger in the belly of our mother we could be in the water? And now that we try to stay longer in the water we can't because we can't breath?

When we were in our mother, oxygen was supplied to us through the umbilical cord, essentially enabling us to breath under water. We no longer have an umbilical cord now, so we can't breath underwater unless we use carry our own oxygen, such as suba diver tanks.

In the womb, babies rely on the mother for oxygen.