It's coming, but you are far more likely to see 3D printed parts than hulls. I don't have the numbers in front of me anymore, but the company I used to
work for did some 3D printed titanium parts. If I remember correctly the printers could generate about 5mm^3 a second or 300mm^3 a minute. At 4.5g/cm^3 thatmeans the machine was printing about 1.5grams/minute, 90grams and hour. Aluminium is about half the weight of titanium, so if you wanted to print a metal boat figure you are printing 45 grams an hour, let's call it 1kg a day.
If the
hull is 10,000kg, that means it will take 10,000 days to print. But it's a large
project, so we could easily use multiple print heads, let's say 100 (at $250,000/head the cost adds up). So that's 100 days to print the hull give or take, and the
equipment cost for it would be $25m.
You could print 3.5 boats a year, 10% ROI on the $25m is 2.5m/year, so you would need to be able to sell each hull for about $700,000 just to cover the machine time. Add in material cost, labor, land, electricity, and other overhead and figure you need to sell a bare hull for about $1m for a 40' aluminium hull.
So in short, possible now, but far from economical. As the
price of machines comes down (and they will) it becomes more and more likely. Particularly for one off and custom boat where the lack of molds will really help.