The OP didn't state if the car wash bay was a typical coin operated installation. If it is coin operated, the pressure pump is usually hard wired and located in a dry location.

If you're plugging in a pressure washer & are concerned about the receptacle getting soaked, install a pendant receptacle that has rubber boots that seal the connectors when you're plugged in.

Regardless of what you do, sooner or later some bonehead is going to see if that wet location connection can withstand the 3500 + psi that these things can put out.

Few things are harder to put up with than the annoyance of a good example.