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Crescent lake can be good depending on weather. I would look elsewhere until the weather gets nasty.The best advise I can give you is to go and watch the birds. You want to put your decoys where they want to be. Go find somewhere ducks are already present and hunt there. If you don't do that, you will have hard time being successful.
Best advice for a new guy... Get a good set of Chest waders. Even if you hunt fields... they are ducks afterall, and eventually you will need to get wet.With a good set of chest waders, you can hunt from sept. to jan. and hunt any body of water up to you belly. That covers a lot of places.Plus you can go jump shooting.
I am not familiar with the areas you're talking about, but will give you some general field hunting tips. Get a layout blind, it will allow you to hide in fields, most have stubble straps, fill every strap with the terrain that you will be hunting in...example, if your hunting corn stubble, fill the straps with corn stubble. Get decent decoys, sometimes you don't have to have 8 dozen, you can still kill birds over a dozen quality dekes. Learn the basic calls and learn to read the birds (this will come with time), sometimes no calling is best. Avoid moving your head around to see the birds, if you can see them, they can see you better.MOST IMPORTANTLY, scout! If you are seeing birds in a certain place in a field in the evening, 95% of the time they will be heading back there in the morning.Good luck.
I would go to Skagit and hunt headquarters instead of mess w/ Crescent lake. Have you thought about Cherry Valley? http://wdfw.wa.gov/lands/wildlife_areas/skagit/Headquarters%20(Skagit)/