I used poly rope last summer when our 110x50 foot pond was being overrun by duckweed and water meal. I left the rope in place but after awhile it started to sink, so i put cheap foam pipe insulation around it and voila! it floated just the right amount. I put 12" zip ties around it (clipped to 4" or so) which helped to keep the pipe insulation on.

You do really need to net out the duckweed once it's corralled, though. When it gets that smushed together, it starts dying off and it really stinks on hot days. (Kinda fun to watch little frogs hop across it like it's a solid surface, though.)