Specific Greek Packing Tips

Well, I was in Greece in late May and early June and did not take any long sleeved shirts. I did take two very light weight sweaters that I wore a few times and on the plane. I did not take any rain gear. I did not bring any pants but two pairs of Capri, a pair of shorts, and a skirt.

My husband took one pair of long pants and two pair of shorts. He brought a very light weight fleece jacket which he only wore on plane.

We did pretty causal things including some serious hiking.

On land in Greece in August you would want only the coolest clothes you own; vests, shorts, cotton/linen, loose cotton dresses, sandals, swimwear. Sunhat. Something down to knees & to cover shoulders (could be a sarong)if you go to a monastery. Then you'll need to add anything extra you might need on the water, perhaps a very thin wrap in case you are on deck in the night, if it's breezy? No long sleeved shirts unless you particularly want to cover up against the sun. Long trousers only if going to the grandest of old-fashioned restaurants

Do remember though that plane will be much cooler. My husband only wore pants when traveling and out to dinner one night. But he was cold on plane with pants and lightweight jacket on overnight flight.

Beth

Many of the beaches are rocky, and several places we swam also had a lot of sea urchins (ouch) on the bottom, so some sort of water shoes might be nice. I took Keens, which were nice enough to also wear to casual restaurants. I'm not certain if Keens are non-marking for the boat deck but I don't remember it being a problem when we sailed. For sailing and kayaking we took a long-sleeved, very lightweight, quick-drying synthetic t-shirt that was sun-blocking. I noticed the boat skipper and guides were dressed similarly. REI has shirts like that for a reasonable price ($40?). We basically used the shirt like a rash guard. You may want a broad-brimmed hat that has some sort of tie that will keep it on your head in the wind. We also bought a little dry bag for our phone so that we could have it with/on us at the beach, and with the better bags in this genre its possible to photograph through the bag under water as long as you don't go deep. Something like this: https://www.imore.com/best-waterproof-bags-iphone, although I would research it first.

For women....Some years ago I did a similar trip on a gulet in October in Turkey and took 2 swimsuits, you will live in them on the boat. Also an old T-shirt to wear while swimming, strong sun + salt water = sunburn! 2 or 3 sleeveless or short sleeved tops, 2 pair shorts, I skirt, midi length to dress down/dress up, I loose shift dress, I nice gauzy top for evening and a long silk scarf/shawl - useful for visiting masques as well as dressing up a bit for evening. Wore heavier clothes on plane including fleece and walking shoes. Keens are great for all purpose walking, etc. Like the idea of using them for water shoes. Plus undies of course. No rain gear.

We travel to Greece using the rule of threes. One set of three in the wash from yesterday, one on today and one for tomorrow. Bring three shirts, shorts,socks, undies, two pairs of shoes sturdy walking shoes and sandals that double as water shoes and of course a bathing suit and a hat. The secret is to have clothes that mix and match so you can change the look. All the shirts are short sleeves but good enough to go out to an outdoor taverna for dinner plus one pair of dress shorts. Honestly you don't need a whole pile of clothes in Greece. We buy a packet of laundry soap and wash the clothes in the sink and hang them to dry on the hotel balcony. Most hotels have a line on the balcony just for this purpose.
On our first trip to Greece we brought so much stuff in large suitcases. At one point we took a side trip to an island and stored most of our stuff and the big suitcases at our Athens hotel. We just took small back packs. Thats when we discovered we never even missed the clothes we left behind. Been traveling with cabin sized backpacks for our month in Greece ever since.

My wife isn't quite as strict as I am and brings a bit more mix and match outfits but her clothes tend to be much smaller than mine and still fit in the cabin sized bag.

Thanks for all the excellent suggestions.