I have one of those for my own use at Shoreline Marina in Long Beach where the boat was once slipped. I do, vaguely, recall being offered the use of one (Marinco plug adapter) at one marina or YC I visited as a transient guest in the past couple of years. You might call ahead if you do not wish to buy one right away to see if a marina you plan to visit has any for the use of its guests.
Actually, I made this one by cannibalizing an old 30-amp power cord. I cut off the end with the female connector, and then put a 50-amp plug on at the other end of that piece. They sell those 50-amp plugs here, but are apparently out of stock.
BTW, this is similar to the admonitions we hear against use of the portable generators/inverters---electrically not really safe. Hasn't done a thing as far as I can see to inhibit the use of those things aboard sailboats!!!
I appreciate the concern for danger and code compliance, but mightn't the warning be a little overstated? 50 amp capacity at the pedestal doesn't necessarily mean 50 amps thru a 30 amp cord. That would only be the case if there is something on board or a combination of things that would draw 50 amps or at least more than 30 amps. In that case, I would think the breaker(s) on your panel should trip. Of course, there is no substitute that can justify sacrificing safety, but I would think practical application would do no harm.
True, I agree there isn't anything wrong with belt and suspenders.
That's actually legal because you are feeding multiple outlets around the house so there is the likely hood that there is not the full 20 amps available to one item. I'm not a fan of it either but I can't argue with the NEC as an authority. My pockets aren't that deep.
The circuit may have a 20 amp breaker
14 gauge wiring is now illegal by the ICC codes in residential housing. You can't even use it as the "traveler wire" on a 3 way 100 watt ceiling light.