All these boats getting 3D printed (also those mentioned in the article) are made from composites, primarily carbon fiber or fiberglass based composites.
Injection molding for plastic parts is easy and fast, however the current process it takes to make carbon fiber composites is highly energy intensive (one needs huge ovens), time consuming, expensive, and some approaches can even be highly labor intensive. This is also the reason why only the more expensive cars in the market have carbon fiber bodies as opposed to all cars/vehicles in the market.
Injection molding for plastic parts is easy and fast, however the current process it takes to make carbon fiber composites is highly energy intensive (one needs huge ovens), time consuming, expensive, and some approaches can even be highly labor intensive. This is also the reason why only the more expensive cars in the market have carbon fiber bodies as opposed to all cars/vehicles in the market.
An example of the process of carbon fiber construction today from Koenigsegg's cars: https://www.youtube.com/watch?v=504I_hJDFck