Captain Nemo and the Underwater City (1969)

When their sailing ship founders at sea, several of the passengers are rescued and find themselves aboard a submarine in the command of Captain Nemo. They are amazed to find that Nemo has built a vast underwater city. Men, women and children all live in harmony in Nemo's idyllic paradise that is free of war and lives off the riches that they've found at the bottom of sea. The new arrivals are distressed to learn however that as they've now seen the city, they cannot leave and must live there for the rest of their days. That doesn't sit well with many of the new arrivals some of whom set about to find a way to leave at whatever cost.