why is ship building industry called one off its kind?

Answer 1

"one of its kind" refers to the ship being built; many ships are individuals, only one ship built to that design; as opposed to mass produced cars, for example..and even when you build a number of ships to the same design, each ship comes out slightly different

Answer 2

Maybe because you have to may certain shapes of the wood or metal frames and stuff and to keep it floating with all the weight on top of the ship and under it.