I think she's a siren 100%. She is ocean / mermaid themed, she sings, she's the adoptive daughter of Signora, a well known villian character who actually stayed a villian rather than becoming good like other villians. Yeah, I think it's safe to say she is definitely a siren.

I also think that because she is a snowland fae, & yk Linnea flies with her head wings, I was thinking maybe Vodyanitsa would be able to fly with her fins. I say this because sirens are also flying creatures!

08.17.26 ,, Just found out not too long ago she is actually inspired by Rusalka, a female water spirit in Slavic mythology! They are absolutely gorgeous, I have been reading a lot about them since I found out actually.. hehe. However, I also heavily see her as a Naiad too, because in game she refers to herself as a water nymph!