In Greece, skipper license is not required if: a) you are the boat owner
you don't rent the boat to others c) the boat has a relatively small maximum speed (eg. it is a sailing boat up to 50ft and has a maximum speed of up to 10-15knots. If you have a larger or faster boat you must have a licence. Also, if you rent sailing boats, organize sailing vacations etc you must have a skipper's licence.
Finally, if you want to rent a sailing boat for your vacation, two skipper licences are required and not just one.
There are a lot of sailing schools in Greece, most of them located in the larger cities which offer a skipper's licence after a two months training and a succesfull examination in the end. The cost is about 400 Euros and the lessons are held twice a week. There is one class lesson and one practical "in the sea" lesson. There are also two weekend excursions where you live inside the boat. The courses are held every three or four months and apart from the greek language there is also the possibility for english lessons if there are enough foreighners for a group.
As for sailing in the Greek seas, because of the increased "sea traffic" and some minor accidents over the last years, the legislation and rules are quite strict. Almost everyone who uses occasionaly a boat for pleasure has been inspected at least once by the local port authorities at sea, for the proper documents and safety equipment. As far as I am concerned, most of the rented yachts fulfill the safety requirements, however, the fact that they are continuisly "misused" by inexperienced sailors suggests that whenever someone is about to rent a yacht he should first examine it thouroughly and never trust the internet commercials and advertisments.