The Undersea Adventures of Captain Nemo was a cartoon produced by Canada's Rainbow Animation in 1975. In the near future Mark Nemo and his nephew and niece Robbie and Chris travel the seas in his sub the Nautilus, solving problems for the United World Organisation. Each 5 minute episode dealt with some aspect of sea life and other ocean themed issues.
Given its low budget(Mark Nemo's character design and the Nautilus were reworked from the 1967 series
Rocket Robin Hood) and lack of composer credits the theme for the series and the background music probably came from a music library.
