Who said that the diorama had to be so large, or that the ships had to be built?Swabbie,
Only a "ship or two!" I feel there are very few modellers these days who would want to make two of three merchant sailing ships that would each be around the 1,500 ton mark, with 17 or more yards in total, to go in a diorama. Here is a typical British passenger liner, and a "windjammer" of about the turn of the century - Sailing ships are relatively easy to build because they usually only have maindeck, poop and forecastle. Steamers are far more complicated because of all the decks, deck machinery, lifeboats, rails, ladders, windows, portholes etc - ...
Nothing wrong with using small scale, simple plastic models of ships.
A diorama is about the diorama (the scene). The ship, and other parts, though they are important, are basically secondary. Of course a great model will enhance the scenic realm of the diorama. The scene of a diorama, especially if it is dramatic, steals the focus of the display. In doing so, the ships become seen but not seen (so to speak).