Hi,

We intend to travel to athens, chania, santorini, milos and mykonos from june 1-15 2017.

Will the sea be too cold to swim in? Or trip includes a lot of hanging around by the beach and swimming. Will we be able to snorkel?

It really depend on your preferences: southern europe in early june is, for me, still a little bit cold to swim in, but I come from a tropical country. Other europeans might find it warm enough. Which region do you come from?

One thing for sure, July and August will be warmer than early June.

about 20 degrees centigrade, may be a bit more in shallow water, less if the sea is waved

We have often visited Greece early-mid June. And only had a few days when it was a bit chilly to swim. Have fun!

How many days are you planning in Athens and the islands each?

Athens deserves a minimum of 3 days to fully explore it.

You haven't stated how you would be arriving in Greece. If a flight you have to decide to go directly to an island or catch your breath in Athens then head out. Remember it's important to get back to Athens at least one day before departure in case of strikes, bad weather or other delays.

Crete is Greece's largest island and would take several weeks to see it all, unless you plan to stay in a specific area for your time.

Santorini, Mykonos & Milos are fairly close to each other but it takes times to get to Point A from Point B, including wait time for flights or ferrys, the trip, packing/unpacking, getting from port to accommodations and so forth.

You may want to eliminate Crete since it's the furthest away from the mainland and the other islands.

I can't help with water temperatures that time of year since we go to Greece in October.

Consider your itinerary and time that is involved from traveling to islands/mainland.

You don't want to waste valuable holiday time traveling so often.

There are already many people swimming in the sea in Crete, & by early June it will be fine. It won't be bath-hot, you won't want to stay in for an hour at a time maybe, but it's pleasant in June.