It sounds like your body was compensating for a long time. It might be decompensating or it could be something else. Just a note, EEG being normal means they didn't see anything, not that the person is without seizures. Some people (with seizures) need multiple EEGs to catch something on EEG (catching a seizure is hard, there are things they might see between seizures, often better at catching something during sleep), some people have seizures too deep to show up on the standard EEG. Something like 20-30% of people with hydro have seizures, it is a higher percentage for certain etiologies.

The fact that they said it was congenital means they saw an obvious malformation, like chiari or dandy walker. Just seeing something does not mean it needs to be operated on, the benefits have to outweigh the risks of surgery.

So good for you for pushing for checking things out! But make sure they really figure out the cause of you blacking out and they don't jump to conclusions as unneccessary treatment can cause serious complications.