The buoy resides at the sea bottom where it is attached by a wire. When your patrol boat passes by above, the buoy releases itself and starts to rise to the surface. At a certain depth the buoy splits into a larger amount of smaller float mines, called mini mines. These mini mines then spread themselves side by side, wider and wider until they reach the surface where they then disappear.
It is possible to destroy the buoy with a depth charge before it has split. An simple buoy may even be overrun without being released, but as the levels become harder the buoy will react faster and split at a lower depth to a larger amount of mini mines.
You score between 2,500 and 4,000 points when destroying a buoy.