This is a musical and a fantasy set in an alternate 1980's Poland- for example, there are no cell phones, but there is some modern technology, no Soviet occupation, etc. This is actually more like a German or early Verhoeven (who is Dutch I think) type of production and set design. It is a wholly original story, half The Little Mermaid if it was done by Fassbinder and written by a Swede (Let The Right One In), with the Flashdance burlesque club,but set in Warsaw. I loved it but it is dark and gruesome, in a delightful way. Interesting characters. Stunning visuals. Satisfying ending. Recommended for 80's kids that watched Art House indies and foreign films growing up. Horror fans that like the unusual will enjoy it.
I was trying to add the 1997 movie She Creature as a similar title, but SLPL only had the MST 3000 version of the old movie by this name. The movie I found similar was originally called The Mermaid Chronicles. Also, in the comments below there seems to be a misunderstanding about why scenes like fantasies during sex and the surgery were not realistic. This movie is about the inner landscape and sexual longing, about the parallels between teen hormones and Siren songs (except these sirens don't drown their male victims, they eat them), not meant to be a realistic movie.