It is true that we would need more facts to answer this question in full, but I think I can shed some insight. Riparian rights means that you own land that abuts the water and that those rights have never been transferred to another. Ownership of those rights means that you have the right to navigate to and from the banks of your property and, depending on the circumstances, the right to erect shoreline protection and possibly a pier. You own those rights subject to certain rights of the...
There are two primary places that I might look to determine whether there might be a claim against the prior owners. First, the contract may have spelled out certain kinds of claims; it may have specified that the equipment was "as is"; or it may have had other terms that would apply. Second, I would look to what you were told about the equipment -- if you were told that it was of a specific kind or quality that it was not, this may be actionable (depending, again, on what was in the contract)...
Generally speaking, the owner of the land is the sole person or entity that can exercise the right to build a pier, and then only with permission from the relevant zoning authorities. It sounds as though you are in effect able to exercise some of the remaining riparian rights -- such as accessing the water for swimming or launching a boat, protecting ones' land from erosion, and generally by preventing others from using the land. In a community situation, what a member of the community...