Safe water marks show that there is navigable water all around the mark and can be used as a centre line, mid-channel or landfall buoy.
The shape of the buoy is spherical, pillar or spar (a round pole shape) and is coloured with red and white vertical strips. The top mark that's fitted when practicable to pillar and spar buoys, is spherical and red.
If lit, it shows an isophase occulting or single long flashing white light. An isophase is a special class of light which alternates eclipses and flashes of exactly equal duration. The buoy shape is optional but should not be in conflict with the buoy used for a lateral or special mark.
Operators of vessels are cautioned that large commercial vessels may pass close by these marks.
On this site
Isolated danger marks
Port closed or channel blocked signal