Unfortunately there aren't many beaches left in Istanbul that I would be willing to go swimming in. Among the few I would be willing to try, the closest one to the city center is Kucuksu beach in Anadolu Hisari. As almost anywhere along the bosphorus strait, currents can be strong, not for beginners. Yet for a cleaner water, Anadolu Kavagi is perfect. There is no beach but the atmosphere is much better. Anadolu Kavagi is the last stop of the (sehir hatlari vapuru) city boat on the Asian side. It's a small yet touristic fishing village.
If you are willing to go further away from the city center, beaches in Buyukada (one of the Prince Islands accessible by city boat) or in Kinaliada may feel more like a resort. Also, up in the north Kilyos or Sile offer clean waters yet need to be cautios as the currents are often told to be strong and the sea is often wavy on the Black Sea coast. Istanbul's swimming season is quiet short unless you are fine with cold water swimming. It starts in late June and lasts until early September.
If you want to indulge yourself in pure luxury and get aquainted with the 'beau monde' of Istanbul, then Suada should definitely be on your list! This Olympic saltwater swimming pool, situated on an island in the middle of the Bosporus, belongs to one of Istanbul's most successful soccer clubs: Galatasaray. Please have a dip, sip your strawberry Marguerita and watch Istanbul's rich and famous from behind your sunglasses.
Craving the sun, sea and some good food, but don't have the time to travel to Turkey's coastal resorts, then Elio is the place to be! This beach club, located on the secluded island of Sedef, is the perfect getaway from the hustle and bustle of city life. Since the beach club is quite remote and hard to reach by public transport, travel in style and rent a private yacht or taxi boat which will take you directly to this little undiscovered paradise...