The boat capsized off the eastern Aegean Sea island of Samos as 60 to 65 migrants of yet undetermined nationality were attempting to cross the sea from neighbouring Turkey, Xinhua reported.
A total of 36 people have been rescued so far during an operation in which Greek Coast Guard and Frontex boats, a Greek Navy ship, a cruise ship and three fishing boats participated.
Seven people, including a child, were hospitalised suffering mainly from hypothermia.
The Greece-bound yacht from Turkey was overloaded and was accompanied by another speed boat with smugglers on board, according to Coast Guard sources.
According to survivors, most passengers were Somalis.
Thousands of migrants seek to enter the European Union (EU) illegally via Greece each year and hundreds perish in such tragedies in the Aegean Sea.
In a joint rescue operation last week with Malta, Greek Coast Guard saved 147 migrants who were on board a vessel in the Mediterranean Sea.