Which sea is better for swimming near Istanbul — Black Sea or Sea of Marmara?

The Sea of Marmara suffers from significant pollution caused by Istanbul's dense population, heavy maritime traffic, and recurring sea mucilage (phytoplankton) blooms. Industrial and sewage runoff make it less safe for swimming.

The Black Sea is objectively cleaner than the Marmara: clearer water, fewer industrial discharges, and no sea mucilage issues. Open stretches and pebble beaches are particularly good for swimming.

If open-water swimming is the priority, the Black Sea is the better choice — it's colder and has larger waves, but the water quality is noticeably better.

The Marmara side offers alternatives to open-water swimming: shaded areas, water parks, and leisure options, for example on Heybeliada island. A good choice if water clarity is not the main concern.