The series was based on the novel "Child of the Vodyanoi" by David Wiltshire. Wiltshire was a dentist by profession, and frequently included dentists as main characters and heroes in his writings, as he felt that they tended to get a bad press.
A vodyanoi (referred to in the series) is a kind of Russian water demon, which drags people down to the depths.
Sgt. Carch occasionally says Scottish Gaelic phrases. These include "Chan eil" (No/it is not) and "beannachd leibh" (goodbye/bless you all).
Location scenes were recorded on lightweight video cameras using Outside Broadcast facilities due to ease of editing. This also enabled shots from the killer's point of view.
This series reunited director Douglas Camfield with writer Robert Holmes. Both were highly regarded for their extensive contributions to Doctor Who (1963), which had included working together on Terror of the Zygons: Part One (1975) and The Seeds of Doom: Part One (1976) when Holmes had been script editor and Camfield was director.