If a call to the maker is all it would take to allay your guilt, I'd say do it.
If it washed away from a vacation home, the owner may not yet be aware of the loss. You might store it for a while and run the CL ad again later in "Lost &Found" as well as Boats For Sale. The cops have your buddy's contact if the owners should suddenly appear, but it seems the salvage rights are his and if he wants you to have it, it's yours.
Full Size Sail Rig
The Kayak Wing
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