Entrance is 350 liras, a great beach with a picnic area, I advise you to visit this place, buy food in advance and come here to relax - the best solution! The beach is sandy, but sometimes there are rocks with overgrown shells and algae on the bottom. Be careful anyway, if you swim, the buoys have a chance to run into them and get splinters
If you want a beach holiday in Istanbul, go to Heybeliada Island. Quiet, nice park, shops everywhere, fenced, clean. There is a descent to the beach, you can pay for admission and use the sun beds for free, there is a cafe.