The buoy has small and large cylindrical sections contiguous with each other and having a common central axis. A hole extends along the common central axis, for receiving a mooring rope. The small section has dimensions to fitly engage into a cavity in the bottom of the large section, such that two buoys are stackable onto each other. In another aspect, the small section has grooves on its outside surface and the large section has notches in its rim. The grooves and the notches are advantageous for lodging a mooring rope and consequently for allowing the stacking of buoys that have their mooring ropes still attached to them.