The Baratie is a high quality restaurant that floats upon the sea founded by Zeff, and this is place where Sanji grew up and works. The Baratie is in the shape of a fish, and has many hidden features--for example, the head can separate from the rest of the ship and be used to fire cannonballs; and the "fins" can be raised to create a platform around the restaurant, so all fighting during a pirate attack will take place there and not in the actual restaurant.
