It's Not Unusual (1970)

Sinopsis

Broke and alone, Dave – a merman – festers in his flat. Seemingly out of options, an advert prompts Dave to take a job opportunity. Without time to think, he is whisked off to Poseidon INC, a mysterious company with unclear intentions. Although dazed, Dave becomes enamoured by the Boss of this organization. This newfound relationship manages to bring some modicum of happiness to our forlorn merman. However, just as quickly as this joy is manifested, it is shattered when Dave realizes what this job truly entails.