Shore Power

Eaton isn't one of the brands I'm more familiar with for these components, but if it isn't a thermal-magnetic breaker, it definitely looks like one.
What is really weird is that "C0,5" indicates (again, with a small benefit of the doubt for Eaton components) that the breaker is supposed to trip as soon as the current flow is above 0.5 Amps.
Which is close to nothing, and surely nowhere near enough to work as the main breaker on a boat like yours.
Mine is of similar size and has a 63 Amps main breaker, if you see what I mean!

Is this a new to you boat? New dock? What work has been done? When did it all last work?
Boat brand, model and year?

Those lil switches are for the meters, I don't think transfer switches or breakers.
If you do not have a manual transfer switch, you may have an auto or remote transfer box somewhere.
Auto transfer boxes are notorious for problems.

Not FIVE, but 0.5!
I mean, half of an Amp, which is a sort of practical joke.
I can't even think what line/usage it could be meant to protect...

Don't take this as anything more than an educated guess, but based on what you said so far I'd hazard the following:
The shore power switch (which judging by the pic of your electrical panel can't be man enough to handle directly the whole current load of shore connection) is likely meant to power a remote relay somewhere, probably not far from the shore power socket.
If so, that switch could power the breaker that is tripping, and downstream of that breaker you could have the line powering this relay.
So, the breaker could trip just because this relay is absorbing 0,5A or more.

Now, it's puzzling that it used to work, and out of the blue it didn't anymore.
But the ridiculously low rating of the breaker could have something to see with that.
I mean, let's say that the shore power relay used to absorb 0.4A while on, and the breaker didn't trip.
But over time, due to a bit of wear/oxidation/whatever, the relay slowly began pulling a bit more current while turned on.
In fact, it takes close to nothing to increase the absorption from 0.4A to 0.5A, hence making the breaker trip.

The easiest way to check this theory would be using a clamp meter and measuring the current flow.
But if you don't have it or don't know how to use it, you could as well replace the breaker with one rated for 1A, and see what happens.
Mind, in principle it's always a bad idea to replace a tripping breaker with one of higher rating, so I would agree with anyone arguing that I'm giving you a somewhat dangerous suggestion...
But I'm saying this because it's totally unrealistic that the wires downstream of the breaker are so thin to overheat and possibly melt just due to a difference of 0.2 Amps or whatever.

PS: don't ask me to refund you the boat if you would set it on fire by trying something you read on the internet, of course!

EATON WMZS2C00
I'm thinking this breaker is a fault/hold down breaker to a relay somewhere.
A dual pole; one pole to hold down the relay, the other to open on over current.
Poor mans GFI possibly.

What new appliances have been installed? Charger, micro wave, anything?
Past this, it may be safer to call in a real marine tech.

The Eaton breaker mentioned is not the main feed breaker. Rated at 0.5A.
Please re-read post #11
Remote auto transfer switches and load relays can get complicated some times.

I remember nightmares on Carvers with similar equipment.
Open relays in a gas boat. It was a long time before my bad dreams stopped. I'll be pissed if they come back now.

I know it is a long shot but have you tried to contact Viking service for advice/ or location of a transfer switch if present? And really out there is it possible to get a schematic of electrical for the boat.
I know that’s wish’n hope’n

Viking Yachts. (609) 296-6000

customerservice@****************