Go to the part where Jack is hancuffed to the pipe, after being accused of stealing the jewelry, and when Rose gets Jack free. Rose cuts the handcuffs off so one handcuff is on his wrist and the other is still on the pipe. If you look, the remaining handcuff is on his left hand. Now if you wait ten or fifteen minutes, or until the boat has sunk, and Rose and Jack are in the water, and Rose is on the board, and Jack is floating next to her, you will see that the handcuff is now on his right hand.