The Greek islands are wonderful from late April to early November. For swimming, sun, and beach activities it's warmest from late May to early October.
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From the warm, crystal clear waters to the stunning underwater scenery, the Greek Islands really are paradise for open water swimmers.
The temperature of the water in any pool is likely to governed by the size of the pool, how deep it is and the time of year you visit. As greek-traveller says, the vast majority of the pools in Greece are only heated by the sun so if you visit earlier in the year they will be much colder than later in the season.
The sea temperature is still warm in October and most days are sunny. Most businesses are open, despite the fewer crowds and, sure enough, you can find great deals in this off-season. The best time to visit Mykonos is October when the crowds have diminished, the water remains warm and rates fall.
The Greek islands are wonderful from late April to early November. For swimming, sun, and beach activities it's warmest from late May to early October.
September is the best month as the seawater has been warming up for some months under a blistering sun. I've often swum in the sea in Greece in early Sept. Remember the later in Sept, the more chance of the weather turning, so the earlier the better. Water temp can also vary according to where you are in Greece.
The warmest region throughout the year is the south-east Aegean: Kos, Rhodes, Bodrum and Marmaris. Except for June and July where also the Ionian, Saronic and Cyclades areas are as warm as the south-east Aegean.
Where in Greece has the clearest water? If clear water is what you're after, head to Balos Beach in Crete, which has some of the clearest water in Greece. What's better, Corfu or Crete? Both Corfu and Crete have incredible Greek beaches.
In the Greek islands, the sun will almost certainly be shining all day and the sea will be deliciously warm for swimming. In the southern islands, you can expect daily highs reaching 28ºC, falling to 24ºC towards the end of October.
By far the warmest waters of Greece: The small island of Kastelorizo. Around the island one can find almost tropical sea life, including sponges, normally not found so far north. Next the east coast of Rhodes. The big island of Crete, further south, has cooler waters, caused by currents.
With average daily high temperatures in the mid-70s Fahrenheit, it's usually still warm enough in early October to sunbathe in comfort and to dine outside at night, with little chance of rain. By mid-October the Ionian and Aegean seas are cooling quickly; if you love to swim, go early in the month.
Kos. Due to its southerly location, Kos is consistently one of the hottest Greek islands. In October, the sea will be a pleasant 23ºC, with evening temperatures rarely falling below 15ºC. Only 4-5 days are likely to be affected by rain.
The warmest sea temperatures in October can be found in Hurghada, Egypt (28ºC), Cancun (27ºC), Dubai (27ºC), Dominican Republic (27ºC), Miami (27ºC), Phuket (27ºC) and Bali (27ºC). In Europe, the highest sea temperatures are in the Canary Islands (25ºC) and Cyprus (25ºC).
Even though there are sharks in the water around Greece, the chance of being attacked by one is very low. Most sharks in the Mediterranean Sea aren't dangerous, and it's very rare to see a shark that could be dangerous. Be careful and listen to what people say, and you can safely enjoy the beautiful beaches of Greece.