There are always cheaper places, but you have to beat the bushes to find them. I always have found non marina dockage. I haven't docked a private boat in a marina since the 1970s. Except as a transient. Check out waterfront businesses and houses with a dock but no marine use. Some people want extra money from renting their dock. Sometimes with power and water and sometimes not.
I have a private dock on the Columbia where I winter. Before I got it, the po rented space for $4/foot and power but no water. I could put in a well, but just use my watermaker.
Old dilapidated docks with commercial fishing boats is another place to look. In general any area that has commercial boats with few yachts has cheaper dockage.
If you're tied to Seattle because of work, you could consider Kitsap County and ride the ferry. I use to do that as a walk on passenger. I kept a beater car on the Seattle side and it was free to park (at that time) on the Kitsap side. Lotta houses over there with docks, many don't use their dock. I use to cross at Southworth.
The further you get away from Seattle, the better prices are in general.
There is a slough or river on the backside of Everett that has some cheaper docks. I haven't been there, but had a friend that kept a commercial boat there in the off season.
Another possibility is get a cheap powerboat and buy dockage for that and tie the big boat to pilings or anchor out. I do that in Alaska sometimes. I carry a 16' Bayliner for grocery trips.
