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Aren't you forgetting that Gollum had an invisibility ring? Anything the Goblins had he could have easily snatched, and they probably did have all the nessesary raw materials for a simple boat, even one made out of wood. I don't find it unlikely he had the skills to construct one either, since he was born near Anduin into a people most likely with boat-building skills. That he was an experienced river-man is also demonstrated by his ability to follow the fellowship down Anduin with only driftwood to keep him afloat. Someone not used to water and boats would hardly be able to do that I think.