I've spent alot of time over the years on San Juan scouting out the beaches for fishing access and fishability. South Beach State Park (next to American Camp, on the South end of the Island) offers the best beach fishing. This scenery on this beach is amazing, definately in my book it's the most beautiful in all of Puget Sound!
I've really only fished off of this beach in late July or early August so my success has been marginal. I've never fished it during the prime time we are in now, so it could be a real sleeper. Try floating a herring 2-3' under a slip float (pull up my post on "Chinookies and ho's off of the beach" if you want detail rigging notes)
As far as beach salmon fishing spots on the rest of the Island there aren't any that are fishable because of the kelp/bullwhips prevent any shore casting. All the boat hot spots like Lime Kiln, Eagle, and Pile Points are loaded with kelp.
Sometimes the seaweed can get a little thick on the incoming high so be warned. Give me a report on your success when you return. Keep your hooks sharp...good luck!
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Bobber Down
"It makes no sense to regulate salmon habitat on land while allowing thousands of yards of gill nets to be stretched across salmon habitat in the water"
John Carlson, Gubernatorial Contender, Sept. 2000 speech at the Ballard Locks