If you mean flat out beach boondocking. I can only only speak generally for parts of Washington and Oregon as I have not been to all of it and IMO even the less than stellar parts are still pretty darn good. In Washington, at least south of Puget Sound, maybe north too, never tried, the beach is a public highway although some sections are closed or seasonally closed. Sections vary but the standard closure period is Apr 15th, (Tax Day) through the day after Labor Day. I've done it on the beach at approximately Greys Harbor but that's been over a decade ago. I know if you get up on the ocean side of the Olympic Peninsula there are wilderness like beach opportunities and IMO the Olympics in general are worth the effort.
As for Oregon largely the entire coastline is deemed public property. That said, access is controlled from no vehicular access to don't get stuck below the high tide line access depending where you are. IMO opportunities for beach driving and camping are better south of the central coast Newport being about the middle. The last time I did it was at Gold Beach which is in the southern portion.
I guess to sum the a-fore mentioned topic it's doable but not as easy as it would seem. That said there are lots of campgrounds in both states that are beach side or nearly so and some public domain to boondock. Every municipality has parks and/or campgrounds, ownership depending on where you are. In Oregon the best known are the state parks, some of the least known are the USFS and BLM campgrounds, though they tend to be a little inland. To wander the coastline and coast range overall there are
loads of beautiful options in the two states. Timing is kinda important with summer and the peak of the salmon and steelhead runs being the high season. Fall IMO is usually the nicest weather and fewer people except again if the salmon are running.
To assist in your planning allow me to offer the following:
http://www.nps.gov/olym/index.htm
http://www.fs.usda.gov/olympic/
http://www.blm.gov/or/index.php
The above link has all the BLM Districts for Oregon and Washington, Spokane District includes the San Juan Islands/Puget Sound
http://www.fs.usda.gov/siuslaw/
The above link should include the Oregon Dunes NRA
http://www.fs.usda.gov/rogue-siskiyou/
Enjoy the trip however you do it and I hope this stuff helps.
! Enjoy!
