Fifty-six people had been aboard the boat cruising in calm waters just 200m from the shore at the time. Some passengers allege that the boat, built in 1924, was overcrowded and short of life-jackets.
Bulgarian newspapers described the vessel, which sank in the shallow waters in minutes, as a "coffin boat" in their Sunday editions.
One survivor, named as Angelika, said: "We tried to hold on to anything, it was awful. I’ll never get on a boat again."
National flags flew at half-staff and all sports and cultural events were cancelled on Sunday.
Gjorgje Ivanov, the Macedonian president, said: "The whole Macedonian nation feels the sorrow for the loss and our condolences go to the families and to the Bulgarian people."
Mass was held for the victims in the city of Plovdiv, Bulgaria’s second city, and Monday has been declared as an official day of mourning.
Eight of the deceased came from one village, Anton. Authorities there have declared three days of mourning starting on Sunday.
If charged and found guilty of crimes against life and property, Filevski could face up to 10 years in prison.