When I was young, I occasionally drove my mother's car, and whenever it rained, water somehow got into the drums. It was so stressful the couple of times I drove it in the rain that for many years after I would have nightmares about being in a car that wouldn't brake. My mother said she never noticed it, and accused me of running through deep water. In retrospect, she was probably correct. I don't recall having any concern about hitting the deep puddles that tend to build up on the sides of the road. I would say that teenage Zed wasn't too bright, but that's probably apparent to everyone since I'm still not too bright.
I think we were all pretty stupid at that age. I certainly did some dumb things and it's amazing I came out unscathed. My buddy used to take us in his 65 Mustang and just FLY down the highway and more than once, he couldn't actually slow down because the drums just wouldn't do the work.
Also, there's a adjusting hole on the back of the drums where you can stick a screwdriver in to loosen and tighten. Mine were previously just open, so yeah, water could get in. But my new hardware came with little rubber stoppers to jam in there to stop crap from getting in.
Maybe they were missing from your Mom's car as they were on mine.