Just a bit of history, traditionally, boat designs are enumerated to ”outside of plank” and it is up to the builder to make the adjustments, usually through the lofting process, based on the actual thickness of the plank.

Have you tried the menu commands,  
First make sure that interior edges are toggled to be visible…then…
Point|InsertPlane
select the Transverse Plane radio button
click OK
…then select the points you just created, in a clockwise direction …then…
Face|NewFace
…and then select the new interior edges and…then…
Edge|Crease
 
This works pretty good, but the creased edges mess up the smoothness of the exterior skin, just a bit.

Another sequence option is:

1 Mirror the outer hull layer using not connected with existing and move it to simplify viewing (half the length might be suitable)
 
2 Move the mirrored hull to a new layer called inner hull (using ctrl-select of the faces) 

3 Mirror the inner hull layer and move it back to the center line (same distance as before).

4 Delete the mirrored part (created in 1-2) of the original as you won´t need it anymore.
 
5 Rescale the inner hull layer with appropriate scale

1-2*thickness/dimension for x and z direction and 1 – thickness/dimension for y-direction. (A 10x2x3 m hull should be scaled 0.996×0.99×0.987 to give it 2 cm thickness)

6 Move the inner hull layer the thickness distance in x and z direction
 
PS If you want the inner and outer hull to end at the same height the scaling in z should be for only one times thickness.