It's in the ferryman's hand, which is clenched closed again. You need to solve the puzzle on his other arm to open his hand again. Read to the end of this hint for a picture.
Before you start, pick up the note on the seat of the ferry for more info. The top part of the note is to Grigor, but the bottom part is for you.
When you look at the puzzle on Charon's arm, Nancy says these are gods, and Grigor's note says "Look to the Romans." Maybe he means the Roman names for the Greek gods.
Grigor's note says, "Take the full order from behind the ancient Greeks." Remember the piece of paper you found behind the "Famous Greeks" puzzle?
That piece of paper (which is called "Letters" in inventory) says J M M D M V N. Those could be the initials of those Roman god names.
Three of the letters are M, but Grigor's note says, "the M order is Mi, Ma, Me."
So the order you want is Jupiter, Minerva (that's Mi), Mars (Ma), Diana, Mercury (Me), Venus, and Neptune.
Okay, you know which order the gods go in. Now use the info from the gods book to move the pieces so each god has the right head, thing in hand, and animal at its feet.
One thing that complicates matters here is that there are two lightning bolts. The big one should go to the god who creates them; the little one to the god who is sometimes allowed to use them.
Another complication is that one of the gods has a golden helmet. This should not be confused with the one who has blond hair. The golden helmet is actually beige and has a crest on it; the hair is just solid gold.
The final arrangement is like so:
This gets you a note that tells you to get eight stones for the judges and some blather about seeds. Save it for later.