From CBS' Famous Classic Tales:
One year after Rankin-Bass adapted Jules Verne's classic tale as a 2-part episode of Festival of Family Classics, Hanna-Barbera took their turn with 20,000 Leagues Under the Sea.
Amazingly, though his voice is easily recognizable in the early moments of the film, the late John Stephenson wasn't credited for his work here. Only three actors were credited, though there were, I'd suspect, substantially more. Also, it's not that hard to recognize some of the incidental music created for earlier H-B series used in the program.