What is the water like in mid may in Greece, is Crete in the southern part warmer?
By Mid May the water in South coast was quite cool. even a bit shocking when we first went in. Once we were in the water a couple of miniatures our body adjusted and that cold feeling went away.
We were swimming in Nafplio and Porto Heli (on the Peloponnese Peninsula) towards the end of May. Water temps were comfortable.
The Libyan sea on south side of Crete is colder than Aegean Sea. So counter to your expectations the water is warmer in the north of Crete . I did not go this past May but my husband did. I did go at Balos which is shallower and thus warmer. In 2018 I swam at Naxos and Samos in May.