Of course humans knew how to swim. Babies know how to swim when thrown into water, and people on coastlines had sure as hell better know how to swim even if, like the Greeks of Hesiod's time, they're not too fond of it. The Persians at Salamis didn't know how to swim, but that doesn't mean people in the Iron Age didn't know how to swim. Immediately when Bronze Age swimming comes to mind the first thing that I think of is the Kadesh Inscription, which shows (on the far right of that picture) Hittites swimming across the river. Many of them are drowned because of their equipment and panic, but you can see on the opposite bank they are coming to land or being fished out (some are being held upside down to get the water out of them)