Technically, On the Waterfront predates the rating system. It doesn't have a rating.
But since it does feature Terry yelling, "Go to hell" at Father Barry, it's not exactly a Disney-level "G." (Dumbo never yelled, "Go to hell" at a priest—though Dumbo does get drunk at one point, which doesn't seem totally G-rated either).
Also, a guy gets pushed off a roof, Terry gets his face bloodied in a fight, and Charley gets shot to death. Charley also seems to be hanging on some sort of meathook, which normally would amp the movie up to a PG-13. But, in this case, it's not really so graphic: thanks to the blood-darkening qualities of black and white film and a general 1950's classiness, this film's suitable for pretty much everyone.