Sinking of Marzanna is an old pagan custom. According to some sources "Marzanna" (called also Mora, Morena or Morana) was a goddess which was offered grains after harvest in hope to ensure good crop the following year. According to other sources she was a goddess or death or winter.
The name of "Marzanna" could originate from "marzec", Polish name for month "March" since the ceremony was taking place in March. It could also originate from the word "marznac" = "freeze, feel cold".
"Marzanna" was prepared as a doll clothed in female cloths who was burned and sunk on 7th of March in Silesia and Big Poland (Wielkopolska) regions. The other dates of ritual of burning Marzanna is the 4th Sunday of the Lent (called also the White Sunday, Black Snday or Death Sunday) or March 21st - the beginning of Spring.
Usually children and young adults took part in the ceremony. Marzanna puppet doll was walked through the houses of the village with the green branch of jupiter in her hand. In the evening she was taken away from the village illuminated by burnt jupiter branches which partly burned her. The burned marzanna was finally sunk in the river.
This ceremony would be probably long forgotten if not an attempt to restore old pagan habits in place of Christian tradition by a communistic government. I remember it very clearly from times when I was a girl scout, 7-10 years old. Every year in early Spring we made a doll from old grass, small tree branches and wrap it in some cloths, then we marched to the river, burned the doll and throw it into the water.
Check Polish Spring and Easter rituals, and the selection of articles about Pagan traditions in Poland.
The Church tried to christianize the tradition of Marzanna and replace it by burning Judah or throwing Judah puppet from the churches' roof on Holy Wednesday. This tradition is cultivated in some Polish regions, but Marzanna tradition is known much better all through Poland.
Sophie Hodorowicz Knab and Mary Anne Knab (Illustrator): Polish Customs, Traditions and Folklore