Half of my AC panel is out

Your missing it.
I will try to explain, again.
A single 30 to 50A, 120vac adapter feeds just one leg of 120Vac on a black wire to an older design 50A connect. Originally designed as just a heavier lead over the 30A leads.
White is neutral and green is fault return or ground. The dock side 120Vac-30A outlet will try to provide service to a cable and boat service that uses up to 50A of current.
Still a single 120Vac lead of two this boat requires.

120/240Vac-50A provides an extra red wire. In normal application, this red wire can cover the total swing of the single phase of 240Vac ( between the red and black wires) or carry two leads of 120Vac @ 50A per leg between the black-white & red-white.
50A on the black wire, 50A on the red wire.
The normal breaker for 120/240-50A service is a double pole breaker. 50A for each leg.
So, in the required adapter and intended installation; red and black are tied together at the adapter, feeding two 120Vac-50A legs of services but still drawing from a 120Vac, 30A outlet and single breaker.

Single pole 30A breakers can only take so much abuse. When they fail, it is not always in an open condition. Then wires over amp,, somewhere.
Please re-read post 46 & 51.

I'm not sure of the electricity on your side of the world. I assume all is 240Vac. Just imagine a white wire drawing from the middle of your 240V and resulting in two, 120V legs.

Look at that carefully, red and black are tied. That is 120Vac to both sides (I think).
Sadly, even the Marinco site is poor on description data.

Or, The Tiara is screwed if the hot side does not go to the chargers. Ha

Thanks for your explanations.
I was actually aware of how 120/240V sockets work in the US, but not of the colour coding.
Which for the records, here is brown for phase and blue for neutral (carrying 23oV/50Hz), with green+yellow for GND.
I'm just a bit surprised to hear that you use single pole breakers for 30A sockets, 'cause here they are always double pole (phase+neutral), regardless of the amps carried.
And they are very reliable in general - I never heard of one failing closed, in fact.

Regardless, why shouldn't also the Marinco adapter that I posted expose to the same risk that you are mentioning?
I mean, if you connect the 30A male to a 30A pedestal socket, but the boat is cabled for 50A (on a single wire phase+neutral, if such thing exists also for 50A wiring), in principle aren't you still risking to fry the 30A plug and/or the pedestal wires in the event of a malfunctioning breaker stuck close, whenever drawing more than 30A?

A breaker for just the hot (red, black, or orange if 3/o) is the usual U.S. supply set up.
The white wire may be dual-polled breakerd, in the boats panel.

Some breaker failures from constant tripping under load;
Builds up an internal flash/carbon trail.
Heat weakens the trip spring.
Usually when they fail, they can not be re-set closed.
I have found tripped breakers that still conduct.
I have found dock breakers that you can not turn off.

It's human nature;
Let see what we can turn on before the breaker trips, POP.
Lets see what else we can turn on before the breaker trips, POP.
Lets see what else we can turn on before the breaker trips, BUUUUZZ, Uh Oh..

I think your example just had one 120Vac leg (black wire). The example, the owner to be offered, had the black and red wire in it.

We are now drifting into a somewhat theoretical and o/t debate, but FWIW I still can't see any logic in single pole breakers on hot wire alone (aside possibly from a slightly lower cost).

Forget the 120 vs. 230 V, that's a red herring.
And forget also "your" 240 V arrangement with two 120 V legs - I'm not sure if there are other reasons behind that, but to me it sounds like something invented just for the practicality of using one plug/socket rather than two.

The point is that the basic logic of single phase current distribution remains exactly the same:
One hot wire and one neutral delivering power, and one earth wire for safety.
I struggle to see how after disconnecting both hot and neutral (but NOT earth, mind!) you could still get electrocuted, regardless of the circuit voltage.

Urgh, THAT sounds scary.
Zero interest in starting an "ours is better than yours" debate, but I can assure you that I never, ever came across a breaker with even just one of the faults you are now listing.
And I'm talking of breakers anywhere from 5A single phase, all the way up to 3-phase, 4 poles, 125A breakers used in industrial applications.

If that's the sort of components that you normally have inside the pedestals in the US, I swallow my previous suggestion that it isn't dangerous to connect a 50A shore power cable to a 30A pedestal socket.
Coming to think of it, I would be nervous even with a 30A cable connected to a 30A socket...!

PS: just for the records, I have a somewhat worse experience with RCD (or GFCI, as I believe you call them in the US) devices, of which I did witness some failures.
But that's a different matter, and connected to leakages rather than load/overload, obviously.

Just one more question, again with apologies for the O/T, but you made me curious:
Do you possibly know what is the normal current breaking capacity of one of your 30A breakers?
Here, 32 rather than 30 is one of the standard sizes, but such difference is neither here nor there of course.
What is relevant is that a typical hot+neutral 32A breaker is rated for a 6000A (yes, six thousands, not a typo!) breaking capacity. If such breaker would still conduct after tripping, pretty sure I'd be pixxed off!

You are confusing specs again and it is NOT relevant.
KAIC is a whole other breaker spec that is way above our topic. Yes, common Squard D breakers have 10KA spec.

I know, and said beforehand, that it's o/t.
OTOH, you said "I have found tripped breakers that still conduct" as if it were normal/acceptable...
That's pretty relevant, in my books.

It is not normal but it happens. Certainly not acceptable.

To be a lil more fair, the environment has to be considered also;
The cheapest breaker on the shelf,
In a wet environment,
Many in a salt environment,
Full of bugs,
And my favorite; Abused by the previous hundreds of users by tripping and/or switching while under heavy loads.

Yes again, abused breakers fail in many strange ways, including not tripping or conducting while supposed open.

Can we put this to bed already?

Well, I'm always happy to put to bed any trouble that doesn't affect me!

But just for the records, I'm somewhat surprised to hear also of your favorite occurrence.
The standard pedestal sockets around here do not allow that to happen, because there is a large rotary switch right on top of it, which aside from turning AC on/off, also incorporates a mechanical plug lock.
In other words, you can neither put the plug it in nor pull it out unless AC supply is off.

You forgot the idiots disconnecting the shore power cord from the boat, leaving it on the dock still powered!! Some of the new plugs have LEDs making it easy to spot

here at Dinner Key Marina, all the electrical was re done after Andrew in 1992. Then in 2005 we had Wilma which brought 18” of water on top of the dock flooding the pedestals. Nothing was done. Then Irma have everything another soaking back in 2017...

They finally just finished rewiring the marina...