Like the other answer, in part I am sure it is literally that amniotic fluid is not water. Your hands will not wrinkle in all fluids, they do in some fluids. I also don't even know if they would absorb water that early even if exposed. You are covered in vernix at birth, and it's waxy & protective. That alone would likely act as a water barrier. You will also notice in wrinkling it's only in the hands & feet. That type of wrinkling is intended to improve traction on slippery surfaces. You don't wrinkle everywhere when submerged, only in the areas where it may benefit you to do so.