1. Don't take ships from the Pirates of Caribbean movie. This means, no Black Pearls, Flying Dutchmans or any other.
2. To create a specific ship type. You must have high enough rank:
At least a Citizen to have a Boat.
At least a First Mate, Sergeant, Witch Doctor to have a Sloop
At least a Quartermaster, Lieutenant, Trapped Spirit to have a Galleon
At least a Captain, Admiral, Sea God to have a Frigate
3. Remember! Frigate is the strongest, then Galleon, then Sloop and the weakest is Boat.