There are indeed places to cool off during the hot summers in Istanbul. Unfortunately, none of the following are equivalent of Maldivian sands or the beautiful beaches of southern Turkey.
Something is better than nothing. Therefore, the following list comprises a list of private and public beaches along the coast of the Black Sea. Kilyos coast is especially pretty popular amongst kiters and windsurfers. Most beaches around there are privately owned by university alumni associations or entertainment groups.
Along the Marmara Sea are a few public coasts. However, unless you are on the islands, you may not really like the idea of the Marmara Coast. Those are very short lanes and the Sea is not necessarily clean.
If you have the means and the time, do go and visit the south of Turkey such as Datca, Cesme and Bodrum. There will be a few examples below from these lands as well.
Nonetheless, pools are often your best choice. Unlike many other cities around the world, Istanbul has very cool pool clubs (almost like beach clubs). Enjoy and cool down.
Kilyos Sariyer Yolu
Beach on island of Halki or Heybeliada as we know it... read more
Summer-style, romantic day tripping with a ride on horse-backed carriage... read more
Galatasaray Adası, Kuruçeşme
Yildiz Caddesi (off Barbaros Bulvari)
Luxurious five-star swimming pool... read more
Bebek Mahallesi, Bebekoy Sok., No:1
Prestige sports club of the Mars Group... read more