When Kongre's men are pulling Denton up using the pulley, the rope that they're pulling should be very tight with the weight of a man, but it's hanging loose.
When Kirk Douglas jumps, the dummy's legs flail around, the knees bending in unnatural angles, revealing it to be an obvious prop.
When being sunk in the end, Kongre's ship is obviously a miniature. Besides, some damage is visible before the impacts (particularly noticeable when the ship is hit in the waterline).
Throughout the movie, every time a cannon fires there is no recoil.
The dead blonde woman in the water moves her eyelids.