So after about 8 hours sailing with the Ghost Ship and trying to figure out how to damage it, we were finally able to destroy it. The ONLY way we could find to get the Ghost Ship to take damage was after it left a zone while visible and entered the next zone still visible. If it enters a zone when it's not visible, it will be invulnerable to damage throughout the whole zone. We even saw a situation where we left a zone and it was not visible and entered a new zone and it was visible, but because the prior zone time was a bit off, the ship was not damageable.
Again, the only way to damage it was if it left a zone while visible and then entered a new zone still visible.
Hopefully this can help someone.
