Simply wrapping a connection with Saran wrap, the kind that clings, will keep an overwhelming majority of rain out. But laying on the ground in a puddle, nothing will protect a submerged joint waterproof yet disconnectable.
I underwent a vertical learning curve designing and installing electrical systems for commercial fishing vessels. I thank the heavenly stars that diesel engines emit an oily vapor when running. The vapor coats everything in the engine room and saves alternators, regulators, switches and contacts from a green death.
How bad? Bad enough that on the bridge or other unprotected area, I cut hundreds of yards of bad wire or contacts where instead of copper conductor inside there was only green powder. Cut the wire and green powder falls to the deck. Where the copper was, a hollow tube remains.
The hotel and RV park I managed had receptacles and switches =yards= from crashing ocean breakers. Go to the edge of the huge patio and look straight down 66 feet and there the breakers were.
Tin coating of conductors only slowed down the process of destruction. The tin would turn black then erode away. Neither copper or tin stood a chance in hell of surviving unless they were protected with a 100% thorough coating with a product that was impermeable to salt air for months or years.
A very rude schoolroom. Marinco sells marine grade switches and receptacles that are twenty times as costly as plain spec grade parts. Their "secret" is to NICKEL plate conductors. Not tin plate but nickel plate all brass components. The hotel retrofit components cost me over a thousand dollars for parts and wire. I could not afford twenty thousand dollars for the same parts.
The process included 30 and 50 amp RV connections at the sites. Outdoor security lighting. At the time RV grade receptacles were flat out unavailable in Mexico. So were spec grade switches and receptacles.
Ordinary grease contains petroleum and sulfur. Smear chassis grease on a buss bar and three months later that buss bar will have turned brown. And the brown is a high resistance coating.
My savior was silicone dielectric grease by the gallon can. With an air pressurized reservoir and needle type applicator I FILLED switches and receptacles. Even the 200 amp main service and breakers were filled with grease until it oozed out. Light bulb sockets, and switches. #6 screws with forked terminals were soldered, meltable inner liner heat shrink applied then smeared with silicone grease.
Bottom line
The steel box for the breaker panel was coated with 3-coats of Rust O Leum brown primer.
And for several years I had zero failures throughout the system. Unprotected fans and appliances failed. Computer motherboards failed, but no power distribution components failed.
I had a wonderful chat with the owners of Mar Rosa RV park in Mazatlan Mexico. The lifespan of their front row breakers and switches was measured in Months. Many failed receptacles were melted which meant many customers had equally melted plugs. I applied silicone grease to a few receptacles. But the 800 miles in a straight line distance between parks prevented me from any followup. Mar Rosa has or had water temps in the high sixties and low seventies during the tourist season. Flores de Las Penas had mid eighties ocean temps. They had humidity in the seventies. My park had condensing humidity.
How does this tie into water protection of a 50-amp plug? If corrosion is stopped, then obviously water resistance has been dramatically improved.
If reducing tension on a plug and receptacle is desired, I had rebar "Y" supports welded up that held 15-30-50 amp cord plugs. Wrap the part that touches the RV cord, with electrical tape then abrasion is eliminated.
I hope this account will help some folks out there understand the effects and cure for corrosion. Remember the saran wrap tip. Six or eight wraps or saran really helps. Leave the last few wraps looser and it will act somewhat like a gasket when facing a solid concrete pedestal facade.