Found this while doing the Looker Side Quest. Not sure if I'm supposed to be able to do this, but whatever. Even when surfing, it still treats it like I'm walking in those four squares so I'm unable to leave the bridge by going up or down. I've tried this on both sides of the bridge, and it works. It's not particularly game breaking, as you can hop back to shore on both sides and walk across the bridge as normal. If it helps, I'm using the Golden Surf Board, not the move.