Plot hole: There are six metal bars mounted around the circumference of the water wheel's axle. In the wheel's interior, when Jack frees himself he lands on his feet, begins to run and promptly hits his head on one of those mounted axle bars, then falls out. Moments later, when Jack returns and runs in the wheel, he is shorter (way more than a foot) than all six of the mounted axle bars spinning with the water wheel, which would make that previous shot impossible, however humorous it is. (01:52:40 - 01:53:55)
Plot hole: In a close-up, just as Jack is freed from the hole in the water wheel, we see the axle's circular wood frame with its spinning bars, above and behind him, as he runs the wheel. At the end of this close-up, Jack's happy facial expression gives way to a look of dread, as he supposedly sees an approaching metal bar about to make contact with his forehead. This is impossible, since all bars are spinning behind him. After a close-up of the actual bar, in the next shot as Jack's forehead makes contact, he stands further back and is now at the center of the circular wood frame, so as to be in perfect alignment with those spinning bars. (01:52:40)
Answer: The jar was to put Davy Jones heart in. He can't touch dirt ever again since he failed to keep up his duties which means he wouldn't be able to get his heart back.