The genius here is in the "bridge" and not the "troubled water." Perhaps a buoy or a raft might be more obvious. The troubles surround and engulf you, but there is something (or someone) to hold on to in the torrent. A bridge, on the other hand, is apart from and above the troubling currents. The bridge is a more subtle and less blatant metaphor, and suggests a better path than merely a lifesaver. While the bridge is more uplifting and therefore in sync with the "Sail on Silvergirl" verse, it is difficult to figure how "sailing on". Fits with taking a bridge over the water. But it works - as poetry. So I'll just leave it alone. As an aside, the text (in its "friend" reference, "shining" times ahead, "sailing right behind", and "easing your mind"), and the swelling (encouragement) in the Gospel music, and Paul Simon's explanation that the "Silvergirl" was his wife who saw a gray hair, all seem to divert from a drug-related interpretation.