It looks like a version of the pepper's ghost effect. The skeleton is actually reflected off of an angled mirror or pane of glass (look at the angle of the ship's mast leaning toward the boat, and the large empty space between the riders and where Jack is standing). I'm not sure if it's a physical skeleton (which would be "standing" on it's side outside of the rider's view) or just a projection, but either way they throw a little sparkly effect on the reflection, change the lighting to illuminate the AA, and presto, we have a Jack Sparrow AA.