As per IALA Buoyage system, Safe Water marks serve to indicate that there is navigable water all round the mark. These include centre line marks and mid-channel marks. Such a mark may also be used to indicate channel entrance, port or estuary approach, or landfall. The light rhythm may also be used to indicate best point of passage under bridges.
What is safe water mark? where placed in a channel?
It indicates that there is navigable water all around a mark. Such mark used as mid channel (Spherical type), Fairway buoy or to indicate best point of passage under fixed bridge.