I crossed chains under my gangway and added turnbuckles to tighten. My floating dock is shorter and wider than yours and still sways (need sea legs, but not so bad). I also added rails to the dock and gangway all around the chains to prevent anyone falling in over the chains. I don't think slides over pipes are attractive either, but I am not sure what other options you have. If your dock has plenty of flotation, maybe hanging submerged weight would slow the swaying, but I don't know. Maybe something like a keel perpendicular to the shore could work, but again, I have never seen it done.