I had taken in a rescue akita once. He spent the first 3 years of his life in a cement kennel in a back yard. Never saw anything. Took my shepherd/lab and him (he affectionately became known as her pet dog) to the river and he fell in love with wading. Every trip, he would go put his big hot paws in the water and take a stroll.
One day, we went to a local park for a walk. There was a duck pond for fishing. Well, Bear-dog saw that water and made a streak for it. He waded right in and....
Oops. Head completely under, we came back up with a look of complete horror on his face. I'm pulling and pulling on the leash to get him out. People are staring in dusgust.
The "duck pond" had been converted from the old community swimming pool. Poor Bear-dog didn't realize that meant there was no gradual change from dry land to deep water.
Then there was the time he realized the pet store santa was an actual person in a costume and not a really big stuffed toy...