A quick search shows that the boat can only be on the waters of the state in WA for 60 days, with an additional 60 day extension prior to being registered in WA state. It says nothing about storing it off of the water.
http://www.dol.wa.gov/vehicleregistration/registerboat.html
Visiting Washington State
Visits of 60 days or less
You dont need a permit to be on Washington waters for up to 60 days, as long as your boat meets one of the following requirements:
It is currently registered in another state.
It has a current U.S. Coast Guard Documentation Paper.
Visits longer than 60 days
If youll be on Washington waters for more than 60 days, you must apply for a permit on or before the 60th day of your visit.
To apply for a permit, visit any vehicle/boat licensing office and bring along:
Your unexpired state boat registration.
Identification from your state.
You may renew the permit once for an additional 60 days. When the renewed permit expires, you must either register your boat in Washington State or remove your boat from Washington waters.