Dave pretty much said word-for-word what I was thinking when I read your post.
There is some hope though. Many manufacturers provide a pretty good overall layout of the boat and where the plumbing
and wire chases go, sometimes even a basic schematic of the factory wiring
. Handy for locating things, but that only goes so far.
What you really need is a library of manuals for all the individual components. These sometimes get stripped during the selling process, either by the PO or the salesman, trying to make the boat look "clean". Or maybe the PO never did any maintenance
and never kept the manuals.
After three years of collecting, I'm pretty much where I want to be, with manuals for almost every component on board, from the main engines to the battery-operated work light. All in both printed and electronic form.
Many are available on line, others you may find if you ask specifically here, or on an owner's forum or other such site.
Good luck. It's amazing how much overlap there is between the boating
and aviation communities. You'll fit right in.