Warlords of Atlantis (movie, 1978)

Searching for the lost world of Atlantis, Prof. Aitken, his son Charles and Greg Collinson are betrayed by the crew of their expedition's ship, attracted by the fabulous treasures of Atlantis. The diving bell disabled, a deep sea monster attacks the boat. They are all dragged to the bottom of the sea where they meet the inhabitants of the lost continent, an advanced alien race that makes sailors their slaves.