Well, first I'd ask what gear have you already got?
Second, what type of hunting do you intend to do most often? In WA, you're gonna have just about every type as an option and, depending on where you're located, you might hunt puddler/dabblers, divers and seaducks all in the same day.
Will you be hunting from the shore/bank or do you have watercraft of some kind?
As suggested by Jeff R. shells are always money well spent. You'll need them most days, if not all days.