I watched the History Channel about WW2 and was amazed at the amount of sailors at Pearl Harbor who could not swim. Many of the survivors stated as much.

Secondly, my uncle joined the US Marine Corp in 1944. He did not know how to swim but thought it was a good idea they taught you in bootcamp. While the "training exercise" time came, it involved throwing him in the pool. He had to be fished out 4 times and in the end they told me he could swim! I guess by walking on the bottom or something.

I know a lot of superstitions were based even upon the premise that you would "catch cold" if you went swimming....probably due to water quality issues in many rivers etc.