I am working on a seastead design.  

The living area or "body" will be out of the water.  For shipping the corrugated culvert 
idea seems good, just bolting together pieces.  The "structural aluminum plate" in the
"box culvert" shape seems good.  This has a corrugated flat bottom and a corrugated side/top section.
It all bolts together.
Lets do 40 feet long and like 16 feet wide and 9 feet high in the middle (sides will more like 6 feet).  
Lets assume 3/16 marine aluminum for now (another choice is 1/10 inch thick Duplex Stainless Steel (e.g. 2205 )).
The corrugated pieces will stack for shipping 
and be assembled on site (as is common for large corrugated culverts).

The body will have internal support frame that makes a rectangle in the bottom edges.  
This is to make 4 "hard points" corners where the legs will attach.  
Part of the reason the body is 40 feet long is so that the beams can be single pieces.
The 4 hard points have to be plenty strong enough as the floats can push up to 2 Gs in
the highest waves.

The front and back of the body will have lots of glass with glass doors.

I would like to have an engineer at a company that sells structural aluminum plate that
I can pay to work with me on this.   It is not the usual culvert job.   Are there
companies that would probably be willing to work out the engineering?   What sort of
price would they charge?