You know, I hesitated to wade into this topic, but there's just so much bad information here I can't resist. As for my credentials, I'm not an electrician or code expert, but I'm an electrical engineer, and have convinced inspectors to issue permits to me. But that's not really important, common sense is.
The rating of a "feed," a marina outlet, for example, which is really just a branch circuit, is the rating of the breaker feeding the branch to protect the wire coming from the breaker to the receptacle. Period. A 50A circuit is rated to supply up to 50A before the breaker trips, and the wire from the breaker to the receptacle is rated to support this. Your 30A rated boat circuit, via an adapter pigtail connected to a 50A receptacle, doesn't "know" it's connected to a higher rated outlet, and doesn't care. Your boat is supposed to be protected against currents that could damage it and your shore power cord in the event of a fault, and should have input breakers appropriately sized. Your boat will draw the same current connected to a 50A outlet as to a 30A outlet if it's a 30A rated boat circuit and there are no faults. If there is a fault, your boat's input breaker will trip.
Adapter pigtails are safe, convenient, and available from most major boat electrical equipment companies, Marinco, for example. Don't hesitate to carry one on board and use it if you are a frequent "docker." (We anchor out, and even so, carry a 50A to 30A pigtail, 30A shore power cord, and even a 15A to 30A pigtail so we can plug into NEMA 5-15R receptacles if that's all that's available. (The conventional 15A household receptacle: this will also plug into a 20A outlet, NEMA 5-20R. 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.