This is five years too late but what the heck...
Diesels do not require any electricity to run per se. On my Valiant (which has separate start and preheat switches) the ignition switch simply applies 12v to the alternator/regulator, to the oil pressure switch, and to the start and preheat switches. The way my boat is wired the instruments are powered by the oil pressure switch--that is, the instruments don't get power until the pressure comes up--no idea why it's wired that way.
If a) you're at all handy, b) not color blind and c) the wires are different colors you could simply check the wire color at the preheater (bolted into the side of the air intake on the manifold). Then look at the small wire on the starter relay. That should give you two of them. The terminal with two wires on it probably goes to the alternator/regulator and down to the oil pressure switch. The power coming into the switch is most likely red, although I would NOT assume it. If you have a multimeter you could switch battery power on at the master battery switch and measure the voltage on each of the wires one at a time. That will allow you to find the 12v that powers everything. The other wires presumably will go to the alternator/regulator and the oil pressure switch if your boats are wired that way and I believe that that's the way Perkins wired the engines.
Then you could use the "OHMS" setting on your meter to figure out how the switch works: In "PREHEAT" position you will get continuity between the switch common terminal and the PREHEAT terminal. In START position you will get continuity between the switch common terminal and the START terminal. In RUN position you will get continuity between the switch common terminal and one of the other terminals. But don't be surprised if the RUN position connects everything together.
Obviously, doing these things will allow you to determine which switch terminal is the common, since each of the positions connects the common terminal to something—preheat, start, etc.
Hope this helps someone sometime.