What will it be powering?
The only thing that I plug on my boat is the Guest charger to top-off the batteries.
Fot that, all that is needed is a standard 16 ga 15/20 amp extension cord as my marina supplies both 30 amp twist-lock as well as 15/20 amp duplex plug-ins.
If you happen to be somewhere that does not supply a 15/20 amp duplex plug, you can buy a duplex to 30 amp twist-lock adapter to power yourself.
I keep an adapter on board just in case I am somewhere where they don't have a standard duplex outlet.
This is the weatherproof plug for the battery charger/maintainer.
It is a standard male duplex plug.
This marina did not have 15/20 amp plug-ins. Only 30 amp twist-lock.
I used my 30 amp adapter on the end of my extension cord and it worked like a charm.