Free: Contests & Raffles.
If it's so close to the private property that any ducks you shoot are likely to fall on the wrong side of the line, then I probably wouldn't hunt it. But if that's not the case, then hunt it for sure. Maybe bring a GPS with you that shows the property lines so you can show that to anyone who says you can't be there.
Just because it could be public doesn't mean you can hunt it either or could be land locked by private. Also the sheriff won't give you a straight answer and say oh yeah go hunt it have a good time. If they did and they were wrong your still in trouble. This day in age of hunting if it has lots of fowl most likely the public can't hunt it so do your homework trying to talk to land owners because ultimately they have the legal rights.
Have you checked out the WDFW website under "Hunting"?I did a quick search for Clark County an found...http://wdfw.wa.gov/lands/wildlife_areas/county/Clark/The website and the Regional Office could be a great help to you.