Depending on what's coming out of the Alternator at that point, it will be providing half wave rectification of the current and/or blocking reverse current.
If there's a regulator in the alternator, then you'll be getting 13 to 14 DC volts on charge with the engine running and the diode isn't needed. (13.6-ish volts with a charged battery.)
If there's no regulator, then it'll be AC coming from the alternator and you should have a lot more than a single diode.
If the alternator is running free without regulation, it could be generating 250v AC! So wear your waders if wet
![Wink ;-)](./images/smilies/icon_e_wink.gif)
As you've been measuring 12 and 11.2 volts DC, there's most likely a regulator in there and I can't see why the diode should be required.
If your instruments are 6v, then you'll need to provide a voltage dropper/regulator to them. "They don't like 12 volts up them Mr. Mainwairing."
They'll read inaccurately at best and probably blow the backlight bulbs if fitted.
All in all - typical boat wiring in my experience. Have fun.
cheers,
S