Which Revell/Monogram
Missouri is this? The two companies - Revell and Monogram - have between them released at least four.
If the one you have is the 1/535 scale version (about 20" long), the lack of a molded-in waterline is, in terms of accuracy and detail, only the first and least of your problems. That kit is a reissue of the very first warship kit Revell ever made; it dates from 1954. It's of enormous interest from the standpoint of the history of the plastic model, but it's about as crude as a warship kit can get - literally. One of the company's problems in designing it was that in 1954 the plans of the original ship were still classified, and the Navy wouldn't release drawings showing the shape of the underwater hull. (My source on this is the bible on the subject of Revell history, Thomas Graham's
Remembering Revell Model Kits. Terrific, fun book.) So the kit designers - who also were concerned about making room for an electric motor and batteries - gave the kit's hull a simple, box-like shape that, as we know now, bears scarcely any resemblance to the original.
For a while Revell sold a 1/720
Missouri, which I've never seen outside the box. I don't think it's currently on the market. On the basis of the photos on the box it didn't look too bad, but the recent Tamiya 1/700 kit is undoubtedly better.
Quite a long time ago Monogram did its own version of the
Missouri, but I haven't seen that one for years. I don't think it's ever surfaced in a Revell/Monogram box.
I think Revell/Monogram has reboxed the Otaki 1/350
Missouri. That one also is generally regarded as distinctly inferior to the competition from Tamiya in the same scale, but probably the best of the four.
There are several ways to mark a waterline on the hull of a ship model. The old, traditional method is to set the hull on a rigid table (being extremely careful to ensure that it's absolutely upright), mount a pencil or scriber on a block of wood that's exactly the right height, and run that gadget around the hull. The more modern (and expensive) method is to go down to Sears or Lowe's and buy a laser level. Such tools emit bright red lines on whatever they shine at; you can level the laser at exactly the right height (it will have a couple of bubble levels built into it for the purpose), level the hull, and use some sort of marker or scriber to mark the hull where the red line hits it. Such a tool will cost about $25 or $30 - and won't be a bad investment. You can use it for all sorts of other jobs around the house; it's excellent, for instance, for leveling picture frames.
Hope this helps a little. Good luck.