Water temp at the end of Sept
 

Brotherleelove, Maybe you would be able to help...
In general, is the water warm enough to swim the last week of Sept on Santorini, Crete, or Mykonos? I know you can't predict weather but what is the norm for the water temp?

yes, it is warm enough.

On Crete I've found you get a thin top layer that's warm but even a few feet down the temperature drops rapidly

I've swum often from various Greek islands in late September. It is warm enough.

It certainly was not very warm in the waters around Santorini when I was there in late September..I would guess in the low 70sF..too cold for me to swim. But then I like warm water for swimming...

A quick Google found this, which includes water temperature:

http://www.greeka.com/crete/crete-weather.htm