I have had several Greek holidays in the first two weeks of October. Temperatures are usually in the 20's Celsius, the sea is nice and warm, but you may need a light pullover, etc once it is dark (but often won't). As a rough rule of thumb if you draw a line tilted SW-NE across Greece, the first places the line reaches (e.g. Corfu) have the wettest weather out of season and the last (e.g. Rhodes/Karpathos) the driest. This is only an approximation as I had a perfect 2 weeks weather in Kefalonia in Oct 2001. On this reckoning Plakias is likely to be one of the best places to go late in the season.
Even if you get a couple of dud days, use them for walking or sightseeing and spend the sunny days on the beach. I think the benefits of early/late holidays for those who can are immense (nicer temperatures, less crowds, lower prices, no kids).