My panel ammeter shows 20+ amps when this breaker is switched on, even with no load. I measure 12.6V at the port cabin lights and fan
If I understand you correctly, with the breaker on (before it trips) you measure 12.6 at the lights and fan, meaning they are not a dead short... This can make it difficult to measure resistance as you'll be dealing with pretty low values.
With all known loads off and the breaker on, I would measure the voltage again very carefully (to at least one one or better yet two decimal places, if your meter can do that) at *every* device on that circuit, drawing a "map" as you go. The voltage should be lower the farther you get from the breaker moving towards the heavy load or short. If there are lighter loads past the problem item, there probably won't be significantly greater voltage drop at those.
It would also help to enumerate everything that is *supposed* to be on that port breaker... and double-check. Could there be any old/unused item that only recently/accidentally got switched on? Or anything that does not work when supposed to? Is there another cabin fan with a seized/stalled motor? A light fixture with a defective socket?
My previous boat had a ceiling fixture connected to both the port and stbd light breakers (apparently from the factory) - we almost never turned them off, but in reality it put both breakers in parallel which wasn't great from a circuit-protection point of view. It also meant things were powered when we thought they were not (with only one breaker off).
If you have or can borrow a clamp-on ammeter (*wunnerful* tools, those), you can quickly check direction and magnitude of the current flowing anywhere along the wire. Hint: turn everything else off, and you can clamp around a multiple-conductor bundle and still get a reading on the one you're interested in.
Also, at 20A draw, you can try just feeling the wires wherever you can - that can tell you when you're (yes, here it comes) "getting warmer" (sorry). You can do this anywhere a fingertip can grip the wire, without needing access to the bare conductor for a voltmeter.
On the bright side, if this was a car you'd being looking for shorts to ground. Assuming your boat isn't steel or aluminum (if it is, look for them shorts), shorts are probably to the return (-12) lead or the housing/frame of some powered device - or the engine block.