I defer to your knowledge of ABYC but I'm pretty sure the NEC does not approve that Home Depot/Lowes/WalMart extension cord for submersion. If the hundreds of thousands of boats using extension cords never end up with the extension cord in the water I'll be very surprised.
Shore power cord-sets from Hubbell & Marinco use PVC jacketed STW 600V 60C wire. This is no different than a a good quality contractor grade extension cord which can easily be sourced with the same STW rating.. STW extension cords are quite easy to find, even in yellow if you want.
From a practical point of view straight blade connectors are not mechanically secure. Ignore the fact that 99.5% (I made that number up - almost all) of outlets are upside down. A straight blade connector not fully seated is a safety hazard. Twist-lock and even SmartPlug are better even if not perfect. I rather like ring terminals and machine screws myself, but hey that's just me. *grin*
Considering how bad the "twist-lock" 30A connections perform, out in the real world, and how bogus the grandfathered current carrying rating is, IMHO they are really only a knats hair better than a 120V standard straight blade.
Again an extension cord is only a last resort when you have a marina "properly wired" for it and only then to avoid a DIY making potentially deadly AC wiring mistakes.. Properly wired for it means GFCI protected dock outlets and, if wired recently, an ELCI breaker at the service point.
That doesn't make it a good idea for unattended applications.
Neither does a twist-lock 30A service, yet thousands of boaters choose to rely on them every day while they continue to cause MILLIONS in fire damages each year.
Agreed. I built a breaker box for my adapter.
And how many 30A to straight blade adapters are out there? Yours would be the first correct adapter from 30A to 15A I have ever seen. I conduct electrical inspections (leakage etc.) on upwards of 200-300 slips per year and have yet to see one. I honestly don't know how they get away with even selling them or how they can sleep at night doing so without the proper over-current protection. Adapters are just one of those "slips between the NEC/NFPA land based standards and ABYC cracks sort of issues.