Its pretty hard to miss, its right above the treasure chest. The only image of this thing I could find was on Youtube (admittedly, I didn't look very hard).
http://www.youtube.com/watch?v=TvrcCjOWyV4&feature=related (I highly recommend watching this with the volume off >_>)
You can see the...
I do remember that mask that changes once you get close up to it. I've always wondered what or who it was supposed to be. My memory of it is pretty fuzzy though, could you post a picture? If I remember correctly, the ghost ship disappears after you clear it, so I can't really go and look at it...