Bring all the documents, survey, deeds and all others to an experienced real estate litigator in your area. It may be a case of either an easement or adverse possession and possible a claim against a title company if you have title insurance when you purchased the property.
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Anytime there is a claim of right to land you cannot sleep on your rights. I would recommend contact a lawyer to assist you with this problem. If you purchased the home in reliance upon the use of the dock, you should inform the previous owners of the situation. You may have a suit against them for the lost value of the dock, as well as, other damages. Good luck to you.
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