A couple of useless facts
Slavic folklore does indeed have a creature resembling "Vodnik", but in Russian, he's been referred as to "Vodyanoy" more commonly. The "ik" suffix in Russian is meant to indicate one's modus operandi, so "Vodnik" means "A man doing things connected with water".
The "yan" and "noy", however, are meant to indicate an association with something, so "Vodyanoy" is simply "A man living underwater".
Also one boy in the episode is called "Albin", which isn't a Russian name, it's actually Latin IIRC.