I believe the local Puget Sound Anglers chapter on Whidbey Island runs (or at least used to) a resident coho netpen operation in one of the lagoons on the western side of the Island. I would guess that's what you saw. There's also a slight possibility it could be a hatchery coho released from a WDFW facility that decided to residualize in the Sound, or is trying to put on some fat before heading out to the ocean.
Unfortunately, WDFW only has a marginal grip on what hatchery fish do and where they go once they're released. The majority head for the ocean, and the small percentage that survive return as adults, most returning to the hatchery, many straying to other systems. But many of the released juveniles may either stay in the freshwater, or residualize in the estuary/nearshore habitats around the Sound. It can be good for the beach fishing, but probably not so good for wild cutthroat, salmon, or steelhead juveniles that may be attempting to rear in the same habitats.