"Although not recommended by medical practitioners, most Greeks could swim according to Herodotus (8.89), who attributes the large number of survivors from the Battle of Salamis in 480 BC to this fact. The 6th-century Sicilian Olympic boxer Tisander used long-distance swimming for training. References are found to both breaststroke and front crawl, while beginners had the help of cork lifebelts and a fresco of the 5th century BC from Paestum shows a youth diving from what appears to be a diving-tower. There is literary evidence for occasional swimming races"
Encyclopedia of Ancient Greece by Nigel Guy Wilson