Will Turner: [Looking at the picture of the key.] This is going to save Elizabeth?
Jack Sparrow: How much do you know about Davy Jones?
Will: Not much.
Jack: Yeah. It'll save Elizabeth.
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 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 spinning axle bars, which would make that previous shot impossible, however humorous it is.
The face of the small music box lid is seen when Will steals the key from Davy Jones. At Tia Dalma's, when Jack nicks the ring, it lies beside a necklace whose pendant face is identical to the face on Davy Jones's music box.