You can supposedly reserve seats and vehicle space online for the İDO Fast Ferryboats connecting Istanbul's Yenikapı Feribot Terminalı with towns on southern shore of the Sea of Marmara: Bandırma (for İzmir trains), Bursa (Güzelyalı), Yalova (for İznik/Nicaea), and a few other ports.
You are required to have a Turkish mobile phone and Turkish citizen identification number. If you don't have these things, you cannot buy a ticket online.
Although the "Buy Ticket" box on the homepage may be in English, after you begin the process, everything will be in Turkish.
So if you can't get your ticket(s) from the İDO website, how do you buy tickets for the ferries?
A: Buy a ticket on the BUDO Ferryboats website and travel with that company. More...
B: Go to the Yenikapı Feribot Terminalı in Istanbul and buy your tickets in person, in advance. More...
C: Ask my friends at Efendi Travel to help. They can get you to Bursa, arrange your hotel and transfers if you like, recommend tours, etc. More...
İDO ferry ticket machines: you must have the same credit card with which you made your reservation, and your PNR (ticket confirmation number), to print your ferry ticket.