Please check the ferry timetables on the website.
Ticket for regular ferry to the Princes' Islands: 5.20 TL with IstanbulCard, 7 TL with token
Wikipedia https://zh.wikipedia.org/wiki/比于卡达
Ferry Timetable http://en.sehirhatlari.istanbul/en/timetable/besiktas-heybeliada-buyukada-3487.html
坐标 40°51'29.92" N 29°7'20.66" E