Large ships are typically launched into the water when the hulls are completed, but that doesn't mean all of the construction work is finished. Usually, there is still a significant amount of work to be done, including finishing the interior of the ship, building the superstructure, etc. Take a look at Image:SMS Seydliz under construction.jpg, for example; you can see the ship is already in the water, but a significant amount of work is being done to the superstructure. You can also read Ship naming and launching for more information about launching. Hope that helps.