mcld
Hi. FWIW I definitely wouldn't call it a bridge, it really isn't a bridge. I don't know of any conventional tagging to express what it is. If I was adding this from scratch I think I would do highway=footway, bicycle=yes, surface=wood, floating=yes. It's really a normal footway but we just want to express that it's a floating structure, that's the only thing that makes it different.

http://wiki.openstreetmap.org/wiki/Key:floating