Any of the mud flats between Mukilteo and Everett are good for digging. I prefer the mud flats next to the oil tanks in Mukilteo. It's a little bit hard to find the access road, but there is a public access where people picnic, beachcomb, launch, etc.

Lots of shrimp there. Clam tube is the best--dig about 4-8 inches.

On the Wash coast, I can usually get them by just using a pitchfork. They are much shallower. Just stick the fork in, and scoop out. I caught dozens within a few minutes. Sometimes it helps to dig when the tide is incoming. Dig right near the waterline--the incoming water seems to push the shrimp further up the holes, and so you do not need to dig too deep.