Hello Harper57,
There are several factors to think about. Sometimes similar ships could have very different lower hull shapes, for example the Iowa and Montana classes. These two are considerably similar at a glance, except the greater L&W and number of main armaments on the Montana. Of course they would be completely different in the view of other more expertised modelers and I do not want to discuss into details as I do not have the knowledge! However, I could say that the upper hull of these classes look fairly similar with the Montana having the external armour belt. Meanwhile, taking a look at the cross section of the two, you could see the lower hulls are very different; the Montana's has a large bulging section, while the Iowa's is completely flat and vertical. Not to mention the different stem shapes and other small details. Some ships hull's underside might not be flat as seen on most. So this is something that needs to be noted and researched well before deriving a similar ship's cross sections.
Even though Titanic and Baltic might not have such a contrasting difference, and their length to width ratios may be very similar, there are still several other factors to think of, such as the propeller shaft arrangements, skegs, rudders, bilge keel, hull plating, water in/outlets, etc. Also, if you were to draw your own cross sections, it will require very careful work to make sure each section aligns accurately with the rest to form a nice curve. I have done this myself with a full cross section and still managed to get a bad wavy surface!
Anyhow, it also depends on how accurate you want to go with your models. Even if it doesn't truly follow the real cross-section, you could still be able to achieve a nice-looking lower hull, which could be accurate enough to be unnoticeable for most people. Plus the fact that many kit upper hulls themselves might be erroneous as well.
I am not an expert in ship hull construction, so some info might be better provided by other more knowledgeable people, but I hope this might be useful, or at least it's just my two cents!
Aop