Bring some warm clothes with you for the night - I caught a cold in my T-shirt and destroyed the 2 of the 4 days being there. Such a pitty. I bought a blouse from a tourist shop when it was already late :(
The Meltemi is a wind that blows across the Aegean from North to South. It can blow pretty much constantly and tends to get worse (stronger) later in the year. During the time that we were in Santorini (June) the Meltemi was blowing as a little more than a constant breeze, whch served to keep you from overheating.
During the day, while out and about, it was barely noticeable as any sort of "nuicance" but at night it was different, as it cooled things a bit too much. Sitting eating on an open terrace facing the caldera could be rather chilly, and a warm top was necessary to keep things comfortable. Some places had transparent plastic "curtains" to help keep the breeze out, and this worked to some degree.
On a positive note, as our hotel was right on the edge of the caldera, the Meltemi cooled things down at night sufficiently that we never needed to put the aircon on to cool the room, and we slept very soundly!
Beware the black sand beaches! Do not attempt to walk barefoot on the black sand if you visit Santorini's beaches in the summertime, unless you're real drunk. Any contact between your skin and the sand can be quite uncomfortable. Even lying on your towel placed on the sand in not recommended. Just spring for a beach chair as close to the water as possible because you don't need beach burn on top of your sunburn.
if going on the beach in height of summer you will need to wear some form of footwear has the sand,pebbles get very hot and can burn your feet