Filmed and set in Halifax, Nova Scotia, Canada (referred to as the "Big City" in the show). A model of Theodore the tugboat can be seen in Halifax's Maritime Museum of the Atlantic.
No voice actors were used in the show as every episode was narrated by Denny Doherty who also performed voices of many characters in the process.
Northumberland Submarine is the Big Harbour's sole resident submarine. During the show's original run, the Royal Canadian Navy in fact had no active submarines in its fleet. Northumberland is not based on the subsequent RCN Victoria-class submarines, given that his dive planes are installed into his conning tower. He bears more a resemblance to the Royal Dutch Navy's Walrus-class submarines, which were new at the beginning of the series.