Many years ago a electrician/boat owner advised me to avoid all plugs for shore power, and to simply hard-wire an extension cord to a terminal block in a locker, and keep the shore power cable coiled up in it.
We have done this for 20+ years
I cut a 2" hole in the transom, where t is sheltered from the rain, and pass the cable out through that.
Some people used a standard elbow-type vent, pointed downwards.
Either hole is easily plugged when at sea, and provides much needed ventilation when in dock