If the white light is flashing, it will tell you what type of mark it is, depending on the flash code.
If the light is steady, it is probably a riding light. Riding lights are sometimes used on a float, to show which way the current is flowing.
Some flash codes (characteristics)
Quick flash. = North cardinal
3 Flashes (groups of 3) = East Cardinal
6 flash then 1 long one = South Cardinal
9 Flashes = West cardinal
2 flashes = Isolated danger
White lights can sometimes be found on mid channel buoys
White lights can sometimes be found on mid channel buoys
Regulatory Buoys.
Regulatory buoys can sometimes have white lights. This type of buoy alert operators of vessels of warnings and regulations in the area.
In the US, to my knowledge, the only regular aids to navigation to have white lights are Safe water buoys (Entrance buoys) & range boards. Private aids to navigation may have white lights also. More to follow.
Mid Channel bouys
White lights can sometimes be found on mid channel buoys
In the United States, red buoys have red lights, and are even numbered. If the are unlit they are in the shape of a semi-cone and are called "nun" buoys. The day markers are red triangles.
Anchor or mooring buoys do not have lights
Safe water buoys are regulated to have white lights on them. Safe water buoys are found in waters close to shore that let boat operators know what parts of a waterway are safe for navigation.
White lights are found on usually indicate mid-channels or fairways. These buoys will usually have red and white vertical stripes.
Mid channel buoys