609 in 2015 and later

648 past in 2015 (updated 17-07-'15)

27# Marathon Gulf Messiniakos, Kalamata-Koroni,(Κολυμβητικός Μαραθώνιος του Μεσσηνιακού Κόλπου) Σ ΚΑΛΑΜΑΤΑ - ΚΟΡΩΝΙ), Greece

Date: 
Saturday, 5 September, 2015 (All day)

30 km, held annually on the first Saturday of September.

http://www.nok.gr/en

info 2014: http://openwaterswimming.eu/node/11752