When Alexander attacked Tyre in 333 BC, would his soldiers have been able to swim? And relatedly, when did swimming become common knowledge among humans?
What an odd question. I'm certain that people have been able to swim since long before we Homo Sapiens were a species. Just for kicks I checked, and swimming is mentioned 12 times in the Homer's Odyssey, which was written @ 1200 BCE.
Perhaps the Greek army was not good at swimming because it had all its armor, weapons, etc. Otherwise this seems a bit silly.