 FAIR: When Elizabeth is on the Black Pearl, she holds the medallion  over the water in order to prove that the pirates actually wanted  it. Though the pirates can walk underwater and suffer no ill  effects, they still jumped when she pretended to drop it because of  the instinct to prevent it from being lost. Breathing underwater is  not a natural human state, and they likely still think mostly as  humans. Furthermore, a tiny piece of gold adrift in the ocean in the  middle of the night would not be easy to find, even for the undead.