A hot water heater is going to pull a lot of amps which means that a connector is going to get hot if it has some corrosion
. Once the connector gets corroded and hot, the best thing to do is just replace it because the plastic gets burned and brittle. You can try cleaning
up the corrosion, but trying to get to the inside of the connector where the corrosion exists is difficult. Does the connector plastic show any dark spots from overheating
where it connects to the boat? By the way, you could try plugging the cord into another dock outlet just to make sure it really is you power cord. Try wiggling the cord to see if you can make connection. That's another way to test for a bad plug besides color. Also, if you do make connection by wiggling, turn the hot water heater on and see where it gets hot to the touch on the cord.
There appears to be a little screw on the side of the connector? You can try taking it out and seeing if you can get the connector apart to clean it up; however, it takes careful scraping and filing to get the job done. Sandpaper does not work very well because of tiny pieces of sand left behind that makes for a poor connection.