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Northwest Seaplanes' is the sister/parent airline of Friday Harbor Seaplanes, not sure which one was technically flying but IIRC they have flown out of the San Juan Islands for years.
As for that many folks on the DHC-3, I think it is a pretty standard load of passengers and no different than the Cessna Grand Caravans that Harbor Air used to fly out of Oak Harbor airport. That was always a fun ride, especially when they went down to mins at Oak Harbor several times before diverting to NAS.