The mini-sub is only used to explore the shipwreck. When the docking lever won't undock it means only one thing -- you are NOT at the shipwreck.
When the sub travels to the shipwreck you'll see a video clip of the trip that shows the wreck. If you saw a video of something else, you went somewhere else. There are lots of little details that could have gone wrong. Post again and give us more to go on.