Istanbul is a city located directly by two Seas: the Balck sea and the Marmara sea. And, it is further intersected by the Bosphorus. As such, swimming becomes quite tempting in Istanbul, but you have to find the right place.
My parents tell me, that it was much simpler to go swimming in Istanbul in their youth. They could actually jump into the Bosphorous from the side of the road, or hire a small boat and stay there on the water for a couple of hours, almost like the fishermen today do. Today it is not that easy as the water has become quite polluted over the years.
[Image By Yusuf fersat (Own work) [Public domain], via Wikimedia Commons]
Nonetheless, here are some suggestion for good swimming possibilities…
If you want to spent time on the Bosphorous, then you have two choices. The first one is Suada Club. Suada is an island in the middle of the Bosphourus, located near Kurucesme. It has its own Shuttleboat service from the landside to the island. It has a nice swimming pool, filled with water from the Sea. But, Suada is not just about swimming leisurely during the day; there are also nice parties taking place at night time.
If you are a bigger group, then I suggest to hire a boat. Most of these boats can be hired by the Arnavutkoy and Bebek coast. You could ask the boat to take you out to Beykoz, Anadolu Kavagi, or Rumeli Kavagi. You shall be able to swim a little around these places (as these are closer to the opening of the Black Sea, which is not severely polluted). The boats usually serve lunch or appetizers.
[Image By Sahrazad (Own work) [Public domain], via Wikimedia Commons]
There are also beach clubs in Istanbul (on both continental sides) around Kilyos (European side) and Şile (Asian side).
There are several different clubs and choices near Kilyos. If you like calm places and the nature, then I would suggest you to go to Dalia Beach Club or GoldenBeach Club. For adrenalin and sea sports lovers I will suggest Solar Beach Club. You will find activities of every kind at this beach club from wind surfing to kiting. There are also weekend parties during summertime, as well as open-air concerts. Shuttle services operate. Detailed information is on their web sites.
Kınaliada is one of the Princess Islands. It is one of the smallest and not to much crowded ones. Kamo’s Beach Club on Kınalıada is a nice, natural and clean place, to enjoy the seaside.
A little out of the city is the Tuzla shore. Dodo Beach Club is one of the best beaches there. Tuzla is located next to the Marmara seaside,and on the way from Istanbul to Izmit.