The extreme northern Istanbul suburb of Kilyos, 35 km (22 miles) north of the Galata Bridge, faces the deep, chilly waters of the Black Sea, and is a favorite getaway spot for sweating Istanbullus on steamy summer afternoons.
Kilyos has hotels, guesthouses and restaurants open during the summer. The beach is open daily in warm weather from 8 am to 6 pm for a fee; it's very crowded on summer weekends, but not so crowded on weekdays.
NOTE! The beach may be subject to deadly riptides and undertow, so educate yourself about what to do in case of undetow (swim parallel to the shore to escape the current, then swim toward the shore), swim in protected areas or where there are lifeguards, and don't swim alone.
To get to Kilyos from Istanbul, take a Bosphorus cruise on a traditional Istanbul ferryboat to Sarıyer (or you can take a bus), then from Sarıyer take Bus 151 or a minibus (Sarıyer-Kilyos) north to Kilyos, a 20-minute ride.