Duct tape does not keep water out. My friend once had an earache so we duct taped the crap out of it. He looked funny as hell and after the surf when we undid it, it poured water out. We got a true lol out of that one.
Something tells me the bond between a smooth plastic surface and tape is better than the bond between something absorbent (like skin) and tape. I have used duct tape on cracks with no water penetration. I didn't leave it on more than a week. The adhesive does break down over time.