Toronto only has about 25 of these in the Bluffers Marina, the city won't allow any more (can't tax a 'boat'). I understand that Vancouver has a lot of them.
Many factors depend if the structure will be exposed to freezing and since I am here and am familiar with this, 2 winters in one. I will work with this winterproof version in mind.
The barges here are floated by 4'X4'X8' blocks of ~1/4" cell white Styrofoam.
Cheers, Rob S.
I'm assuming you will use you giant gantry to 3D print the narrow boat, but with what? cement and strands polypropollene for reinforcement, Kevlar or Carbon fibre. I'm quite curious.
A floating dock with rails, so that the gantry can move along the dock while it builds the boat.