Actually, even though the pedestal on the dock is a branch circuit from the marina the cord to your vessel is considered a service entrance just like the service on your house....As long as you don't draw more than the branch circuit provides, you will be fine.) So, the marina's branch circuit breaker protects their circuit from breaker to receptacle; your boat's input breaker protects your boat's AC input wiring, including your shore power cord.
That said, I'm open to hearing a substantiated argument for why this isn't so, or safe. I know I can learn something new every day, and I'm all ears.
The problem begins right after you put in the adapter and then connect equipment that is only rated for 30 amps to the 50 amp adapter and breaker. Now your putting 50 amps of potential power down a 10 gauge wire system that is only rated for 30 amps. That is way under rated and can easily cause a fire if there is a fault, long before the breaker trips.
Your input breaker, or your first main breaker in your vessel is the first place that has now started to protect the vessel at 30 amps. Up until this breaker every current carrying device is at risk for overload and fire. The 50 to 30 and the 30 to 15 are both against code and are a hazard and both are covered in my patent.
When using an adapter on my vessel, and I do use one occasionally, here is a list of what is in jeopardy.
The 30 amp shore power cord, the shore power plugs on the side of the vessel (where fires have started), the wiring from there to the 30amp rated isolation transformers and the transformers themselves and then the wiring from the transformers to the main disconnect breaker in the aft cabin. From that first 30 amp breaker you are back in compliance but there is a lot of room for fault. Also breakers don't trip as soon as you hit Xamps plus .01. They have a curve based on heat over time. Technically you are suppose to "continually" pull only 80% of a circuit's rating which would mean that on a 30 amp circuit you should only be pulling 24 amps under full load. I know the boat builders don't calculate to that spec. The problem really lies in the fact that without a properly sized breaker to protect you from over current you might not be fine. They don't call it a fault for nothing. Wire chafe, loose connections or a short circuit of any type can lead up to a fire especially when there is more current available than the circuit is designed to operate.