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://ru.wikipedia.org/wiki/Бююкада
Ferry Timetable http://en.sehirhatlari.istanbul/en/timetable/besiktas-heybeliada-buyukada-3487.html
Координаты 40°51'29.92" N 29°7'20.66" E