Good question, not easy the answer, unless you travel around with the latest official report on clean & dirty beaches based on extensive water tests.
Keep Away when…
…the surface of the sea is iridescent; it most probably contains motor oil, petrol oil, sewerage and other things you don’t want to know or swim in.
… the sea is deep green. This color comes to 98% from rotten seaweeds and plankton.
… the beach is very dirty. Most probably the dirt has landed to the sea too.
… there was a heavy rain. Heavy rain can washes all possible raw sewage from earth suface and force it into the sea.
… there is a sewage plant or any other plant in the area.
… the water is clean but there is a bad smell around, even a slight one.
****** My father used to tell me that good sign for clean water are… sea urchins. They hate dirty waters, they don’t even come close.
You common sense will tell you not to swim with ports or yacht marinas where small and big boats threaten your life directly, right?
After having swum in unknown waters, do not forget to clean your self , your bathing suit and towel with soap and plenty of water.
Jellyfish came too close to you? If no liquid ammonia around, put some clean sand or clean (vivid green) seaweeds on the spot.
Living in and around Athens?
The Pan Hellenic Center for Ecology Studies has issued this years’ report about clean and dirty beaches in the broader area.
Unfortunately the report is in Greek, but with the help of the maps and beaches numbers you might understand which one is A (dirty) or K (clean)