Rockfish, Downriggin
I've been working with WDFW observing seals at the river mouths of some Hood Canal streams for the past 5 years, and we would see seals coming into the rivers and going above tidewater. Most rivers on the canal are low enough this time of year, that they can only make it in on an incoming/high tide.
Downriggin, you are correct, seals in Washington are doing well. They experienced dramatic growth after the passage of the Marine Mammal Protection Act in the mid 70s, and now appear to be leveling off. I have not been directly involved with impacts on herring, but you are correct that both harbor seals and Calif sea lions are believed to be a contributing factor. Those I've talked to about the Cherry Point herring stock feel that seals and sea lions most likely did not cause the decline, but may be a factor in preventing recovery now that the stocks are so low. If you have further questions, email me and I might be able to get you more info.
Cheers
Josh
PS, you sure that wasn't a river otter on the Union river?? While sea otters have been seen on a few rare occasions in Puget Sound, they are almost exclusively found on the outer coast and just inside the strait. River otters typically swim with just their heads out of the water and have long tails. Sea otters will swim on their backs and have very small tails.