Yes, the law in Washington State states that navigable rivers have the property owner's rights end at the normal high water mark. Which means, as you said, that fisherman, or anyone else, can use the river by boating or wading/walking up to the normal high water mark and be perfiectly legal.
These folks I spoke about ay nothing to boaters unless they park on the gravel bar and fish through on foot, which is how I prefer to fish my boat is just transportation. Then they or their spouses come out and start the harassment. In fact, one particularly obnoxious landowner on the Sol Duc will come out on to the gravel bar and start shooting his shotgun all the while telling you that he wished he could shoot the 'tresspassers and not get arrsted for attempted or aggrevated murder'. Sure makes fishing there less than enjoyable.
A good friend of mine in Port Angeles and I don't allow him to run us off; but it is not a fun experience having him blasting the shotgun next to you including over your head. We reported this to the Clallum County Sheriff's Office multiple times and got the same response, namely why don't you guys just go somewhere else and let him be. Afterall, there is a lot more river to fish. Don't you love law enforcement when they don't want to do what is right and have a talk with landowners doing this crap.
You are correct about smaller rivers, streams, creeks, they are not considered navigable waters and the property owner can own the whole stream bed from bank to bank or to the 1/2 way accross point if someone else owns theother bank.
Yes, the landowners harassing fishermen are breaking the law; however, I have found that Sherriff's and Sheriff's Deputies to have no interest in telling the landowners this. Instead, both here in Skagit County and over in Clallum County the Sheriff's Office listened to me and then told me that I would probably be better off just fishing somewhere else to avoid trouble. Seems like they don't want to risk angering a landowner.