This green buoy is one of a series that marks the north/west side of Buttermilk Channel. The green color and odd number (5) indicates to mariners that they should keep it on their port (left) side as they are heading north in the channel.
This image shows the buoy tilting to the north, a visible effect of currents. The photo was taken on 8/9/16 at 12:15pm, as the current was at its maximum flow speed of 1.5 kts [see image of a graph of currents for that day].
See In the Water for more information on navigation in the harbor.