Grays Harbor would be a first for me. I wouldn't say they are not there because those birds can relocate 50 miles in a year. About 4 or 5 years ago, we released 300 birds up and down the I-5 corridor - some to enhance flocks, some to start up flocks. I know some of the start ups are doing well, and I actually saw a hen walking along the water in Everett just north of the Naval Base. Blew my mind.
You are correct on the spring season - April 15 to May 15, but you can also put in for a special tag that would lengthen the season if drawn.
As far as loads, some swear on 4's some on 6's there is even a 4/6 combo that works great. Main thing is to pattern your gun and see which load works best for you.
Starting with calls - I would suggest a box call or push pin to start with, and then slate, and then a diaphram. The merriams to the east seem to just love a box call.
Tactics range depending on the weather, the terrain, and the bird. The neat thing about hunting turkey is as soon as you have them all figured out - they do exactly opposite of what you would think they should do. You can find some hunting information on Washington turkeys at
http://www.wanwtf.org.
One of the best things to do is hook up with someone who is familiar with turkey hunting - Most turkey hunters are happy to take someone under their wings and teach the tricks to the trade. Find a local NWTF chapter and get plugged in - you'll hear places rumors of places and other things that would be beneficial to your hunt.
Small warning though - when you hear that booming gobble echo through the woods there is a VERY good chance you are gonna get hooked. And if you happen to call one in - forget it, the only way to get out is join TA (turkeys annonimous) but I don't think they exist yet. Good luck
http://www.wanwtf.org