The surface marker buoy (also called safety sausage or SMB) is an inflatable buoy with a line attached, that enables divers to visualize their position on the surface. Usually it is filled with air from the alternative air source during the safety stop on 5 meters and sent to the surface. So divers can make sure before surfacing that boats or jet skis notice them and keep appropriate distance.
For drift dives, the surface marker buoys are particularly marked so that the captains can easily identify and pick up divers from their boat.
Furthermore, they also serve divers on the water surface to improve their visibility and discoverability, especially in times of high seas or strong currents.
Use and function of an SMB is one of the diving skills learned in the PADI Open Water Diver course.