OK. I've tried to model the main dock. What I've done :
a) imported a basic rectangle from harbour_kit.dat
b) de-triangulated it
c) selected both edges (top and bottom) to cut them into several parts
d) moved the new vertices to the correct place
e) connected each vertices (top and bottom)
f) triangulated again
And how it looks like :
But now, when I try to import the model into S3D, an error message says :
"One or more vertices is not mapped to a texture coordinate."