Gibbs: So we're setting out to find whatever this key unlocks!
Jack Sparrow: No. If we don't have a key, we can't open whatever it unlocks, so what purpose would be served in finding whatever needs to be unlocked without first having found the key that unlocks it?
Gibbs: So, we're going to find this key?
Jack Sparrow: What good is a key if we have nothing for the key to unlock? Please try to keep up!
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 considerably 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.