You can reach directly the Marmara Island from Istanbul (Bostancı and Yenikapı) by the fast craft (İDO). Through the summer season there are 4-5 trips everyday in both directions. By the month of april there are only weekend trips on fridays (from Istanbul to Marmara) and sundays (from Marmara to Istanbul).
There are daily ferry boats from Tekirdağ to the Marmara Island. You can heck the timetable on the web page http://www.marmararoro.com
There are also at least 3 ferry boats daily from Erdek to the Marmara Island. You can check the timetable on the web page http://www.gdu.com.tr