In reading Annwyn Avalon's books, which heavily reference Celtic folklore, I was reminded of the many bits of Slavic lore around water spirits. The first that came to mind was Syrenka Warszawska (pronounced phonetically Seer-ehn-kah Var-shahv-skah), or in English, the Mermaid of Warsaw.
Her history is that of a siren, but she's actually a melusina, a fresh-water mermaid. There are MANY tales of the Mermaid of Warsaw, the earliest dating back to 1390, however, the most popular tale is that the mermaid originated in the Baltic Sea, swimming down the Wisła River to the shores of Warsaw. She enjoyed the area and spent time making mischief and singing--her beautiful voice enchanted the locals so much that they put up with her pranks, but supposedly a rich merchant tried to trap her. Legend says that the local fishermen were able to save her from the trap, and in return for their kindness, she took up sword and shield as a protectress of the city.
Warsaw takes her very seriously (as they should!) as motifs of her are found throughout the city, in addition to being on the city's coat of arms. Of the numerous statues around the city, this one near the Świętokrzyski Bridge is my favorite. It was made in 1939 by sculptor Ludwika Nitschowa and is one of the few monuments in Poland that survived World War II, despite the the rest of the city being deliberately and completely razed by the Nazis simply out of spite in 1944. To me, she represents the resilience of those who are fighting for a better world, one of kindness, justice, and freedom, raising her sword high with strength.