Aglayankaya is a pretty little beachbreak north of Istanbul. While the waves aren't what you would find on the ocean, surfers can find worthwhile waves most of the time. Do keep in mind that the water gets quite cold in the winter time. Crowds are rarely a problem at Aglayankaya.
Just across the Bosphorus from Istanbul and on the Black Sea coast, Merkez Sile offers very regular waves. This beachbreak has the longest waves in Turkey, extending for 300 meters on a good day. Access is relatively easy if you rent a car.
Kumcagiz is a sandbar that is recommended for all levels of surfing expertise. Kumcagiz is a couple dozen miles to the east of Merkez Sile. The weekend crowd can be a bit of an ordeal, so be sure to show up during the work week when things are a bit more calm.
Igne Ada is another sandbar on the Black Sea, this time northwest of Istanbul, close to the Bulgarian border. The swells can be up to 5 feet and wave jockeys can access the beach easily just by walking there from the town of Igneada.