Here is the complete HTML file containing the 3D WebGL scene built using Three.js. It precisely follows all the dimensions, placements, and structural descriptions you provided.
To view it, simply save the code below into a file (for example, `seastead.html`) and open it in any modern web browser. You can click and drag to rotate around the seastead, and scroll to zoom.
```html
Seastead Trimaran Concept
Seastead Design Concept
Left Click + Drag to Rotate | Scroll to Zoom | Right Click + Drag to Pan
```