Probably you don't see any mention of swimming because the water is too cold:
The average year-round water temperature is between 48 to 52 degrees.  Warm enough if you are a member of the polar bear club but too cold for most people.  Several of the parks do have water access so you can explore tidal pools and such and maybe splash some cold water on yourself to cool down.

Orcas Island has Cascade Lake in Moran Park that warms up enough to go swimming (circa 70 degrees by late summer) but that's the only place I've seen anybody swimming.

Ron