She had to have been misdiagnosed. Because how does a near drowning cause infertility?

Agreed.

Perhaps it had something to do with her getting hurt by the onset explosion? If she greatly hurt her lower body...

Actually, it is true scientifically that falling on the back affects the ovaries. And she fell on her back as it was shown. And such a fall on the water hits hard when you plunge flat, on the back or stomach, without creating an opening on the water surface. Not to mention she was thrown not even plunging. Think about when swimmers plunge, they always go with their hands or feet first to create an opening for their bodies to enter the water. And they either plunge vertically or sitting but never horizontally, it can kill sometimes.