The movie is in color and probably from the 1960s. I don't think it is Mysterious Island or Captain Nemo and the Underwater City.
The only scene I remember involves characters in a room, with wooden planks on the floor and the walls. The center is bare and prisoners face a group questioning them.
They are wearing what looks like pirate or sailor costumes from an earlier century.
At the center of the room is a trap door. After questioning one of the prisoners, one of the men opens the trap door, revealing the mouth of a monster. The prisoner is thrown into the trap door and is eaten by the monster, feet first.