What do you call this device that floats in the water at ports?
I've seen lots of these, at ports, floating on the water:
I don't have a clue about what they are called.
light(ed) buoy
Images of lighted buoys
lighted buoy
Floating object used as beacon. Visual Dictionary illustration
light buoy
(Nautical) A buoy with a light on top which is lit at night to aid navigation. Wiki
buoy
A floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents. Wiki