Across the bay from Seattle, Alki used to be a dumping spot for garbage, back in the day. Now the combination of decades past trash and Puget Sound currents have made Alki Beach a popular place to hunt sea glass.
While Rosario Beach is better know for for sea glass hunting, you can also make some great finds along the Deception Pass beaches and get in some great views at the same time.
The beach at Coupeville is a bit of a secret location, but anywhere a town sits right on the water, you can bet that garbage got thrown in it.
While this beach get picked over pretty hard in the summer, winters storms continually churn up new glas, and you get some awesome beaches to wander on.
Whidbey Island in general is a great place to hunt sea glass, but you may want to make Bush Point one of your sea glass hunting stops.
Another great beach to score some of Puget Sound's sea glass as well as agates. In addition there are loads of trails and other fun thing to see like the bridge in the nearby area.
It takes quite a hike to get out to the good spot, but Glass Beach just west of Port Townsend, but it is usually worth the hike.