Hungarian MAROS, river, rising in the Giurgeu Range in the Eastern Carpathian Mountains, east-central Romania. It cuts a gorge between the Caliman and Gurghiu ranges, crosses the Transylvanian Basin southwestward, and then cuts across the Western Carpathians between the Poiana Ruscai and the Bihoru mountains and emerges onto the Tisa Plain to join the Tisa (Tisza) River at Szeged, Hung., after a course of about 450 miles (725 km).
Bosnia and Hungary do not have a common border then or now. So this river must be the one you are refering to.
As for a Serbian name Bosco. I have never heard this for one of Serbian descent. Perhaps is was Bozo (sounds like Bozho).
Plavi means blue, such as Plava More (blue sea) or plavi plamen (blue flame). The -ic ending is a diminutive term much like -son in Peterson or Mc in McDonald, all of them mean son of. So Plavsic would be son of blue.
Zora is dawn. Stefan would the Serbian version of Steven.
Croatian Heritage Museum