I can't agree more with Moose (Al) more about needing to understand the principles of boat wiring.
The paradox is that if you really do, the diagram is unnecessary. However, it would nontheless be a useful tool in the learning process.
Another problem is that there will be variations within boat models. Not all Cd-22's, for instnace, will be the same over the years. And even within a year, there may well be variations and changes. Optional equipment installed at the factory will add another dimension to trying to keep track of everything.
Add to this the many changes made by the installer/dealer or previous owner(s), and the diagram becomes only a general guide to what SHOULD have been done.
I once asked for a diagram of the Fleetwood Limited motorhome we have (the underside of the dash looks like a B-52) , and was told by Fleetwood that they didn't have one, that the components they used varied from day to day based on supplies, and the resulting wiring couldn't possibly be standardized.
Our C-Dorys may well not as bad as RV's, but you get the idea. Mass produced automobiles, on the other hand, have to have so much planning to allow assembly line production that a diagram preceeds everything and must pretty much be followed. Good for them.
It is also interesting that many journalists who write for boating magazines partially judge the overall quality of the boat by the "cleanliness" and organizaton of the dash and other wiring. Sloppy does not impress!
No wonder several members of this site have pulled out much of the wiring behind the helm station and totally reorganized the Great Spaghetti Casserole of Copper Wires that lies therein. Dave (oldgrowth) is the latest example. 3rdByte (Mark) did this on his previous boat (2nd Byte), and it well have been the salvation to sanity that allowed him to survive one of the Alaskan winters.
The best outcome of this thrust, however, would be to get the manufacturer to put in larger fused buss bars with both already dedicated / labeled circuits and many open ones that could be convenientlhy penciled in as the boat was subsequently modified. Not a bad idea at all, actually!!!
Joe.