I do not know the specifics of Washington law (check with an attorney there) but generally time for a prior owner is added on to the adverse possession period. (It is more likely that the adjacent property owner would claim a prescriptive easement than adverse possession.)
In Washington, one of the required elements for establishing either adverse possession or a prescriptive easement, is continuous use of the disputed area for a certain period of years which, with limited exception, is typically 10 years. However, under the doctrine of "tacking," the 10-year period does not need to be continuous possession by the same person. Instead, a claimant can "tack" their period of possession onto the period of their predecessor in interest in order to make up continuous possession during the statutory period. Thus for example, if the immediate predecessor in interest used the disputed area for 9 years, and their successor in interest continued that use for 1 more year (without a significant interruption in between), the periods could typically be tacked together to meet the 10-year requirement.
This answer is provided for informational purposes only and should not be relied upon or construed as legal advice or legal opinion. The answer is based upon limited facts and general principles of law, both of which are subject to change and, therefore, may materially affect any answer given. You should contact an attorney to obtain advice with respect to your particular issue or problem. This answer does not create an attorney-client relationship, nor does it create any kind of legal relationship, duty, or privilege. This attorney is licensed only in the State of Washington.
I agree with both previous answers and you need an action for ejectment to remove the docks. Take your lease and all of your paperwork to an attorney right away.
Only If and until you and I sign an Agreement for Legal Services, I am not your attorney. These answers are provided for informational and/or novelty purposes