White lights on buoys

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