Fear of water
Peter had a fear of the water that he didn’t really share with his parents, but they all knew that he was afraid of it. He had seen a lot of scary movies as a kid and he knew that underneath the water, there always was the chance of something lurking around.
In reality, though, the chances of something happening to the average swimmer are highly unlikely, unless, of course, you jump into a pool of sharks. But even then, most sharks can’t be bothered to attack a human being, pop culture has given them a bad name. Peter was too young to realize that yet though.