I like the prop idea. Its more feasible they flew her in on a plane that lands in the water and put her in the Parked Sub. she would have been none the wiser.

Also. about LOCKE knowing... He did walk back from the Sub all wet. which ment he went into the water. He probable saw it was a prop and dove in to check that its propellars didnt exist. When he sees it isnt real he blows it up anyway, to start some civil unrest among Henry's people. That way he is still in control of whats going on. As opposed to Jack finding out it was a fake. IF Jack found out it was a fake then there is no way that he would trust Juliet and bring her back to the camp like Henry wants.