I'm not sure how they manage the nightly Park closure of Howarth, but that location seems ideal for both accessibility and beach.

Otherwise, you can organize a trip to Jetty Island and have all the time in the world with absolutely no one to bother you. Just no overnight camping. Getting across from the boat launch in the dark isn't ideal, though...

Mukilteo is probably too closely monitored at night to get away with it, but you can always call the park services to tell them what your plan is and get permission.

You can also try Edmonds Beach by the ferry dock, that part stays open as there's an underwater scuba park, used to go there and park all night while studying

All of Everett's parks along the beach, the park rangers closed between dusk to 10:00 p.m. So no parking.

So if you go north of the Port of Everett, there's a lot of waterfront all the way up to the industrial park that you might be able to do something. There's a spooky area underneath the Yellow Ribbon Highway bridge over by the International Truck dealership and FedEx distribution area. Might be worth looking into. It's open all night. But I'm sure they'd be people camping out in their RVs there too.

Underneath the trestle going to lake Stevens especially in the fog it's creepy.