I think a ship's model is "correct" when its general dimesions, shape (hull lines, funnels), tonnage and speed are like the real ones.
(Obviously, the hull lines of the SH4 model of Hog Island were not correct, and some other details (the rudder for example)). Then, you can add different customized details for the ship.
For example, you can add different options ("loadouts") for various AA guns arrangements and platforms.
Ideally, each merchant ship from the same class should have a different appearance. For this, you can play with skins (hull textures), 3D details, and guns arrangements.