[iknownothow]

You are correct.

I'm not sure why I remember cement to be water proof. Its possible I infered from how its been used around me. I have seen cement very often used where water contact is expected (bathrooms etc). I have also seen brick walls plastered with an extra layer of cement as putty considered to be rain proof and any leaks were thought of as poor workmanship rather than poor material choice.

P.S. I'm not a builder. I've taken an interest in the topic a few months ago.

[dnemmers]

Hydraulic cement is made for some of those occasions:

https://www.quikrete.com/productlines/hydraulicwaterstopceme...

But I wouldn't want to make any sort of building that only relied on it for waterproofing.

[Groxx]

Might be because it's water tolerant, i.e. it doesn't break down with exposure to water? Roman über-concrete stories are always full of "check out the concrete that's sitting in sea-water, it's still going strong! maybe it even helps!" cases too.