A boat buoy, whether used as a marker buoy, a boat fender or a mooring buoy, has at least one ring to which you can knot a rope. The key is using a knot easy to tie and untie that isn't likely to come untied by itself. Understanding how to tie a knot on a boat buoy, though, means you need to know a few terms used when tying knots, like "bitter end," for the very end of the rope, and "standing part," for the part of the rope that's there, but not part of the knot.
Push the bitter end through the hole or ring at the bottom of the buoy. Pull 2 feet of rope through the hole and push the bitter end through again, pulling tight the rope wrapped around the ring. Repeat this, without pulling the second wrap tight.
Pull the rope's bitter end around the standing part of the rope, making a loop a few inches away from the buoy's ring and push the bitter end through the wrap you didn't pull tight.
Loop the bitter end around the standing part again, until you can tuck the bitter end underneath itself. Pull the bitter end to draw the "Fisherman's Bend" knot tight. Wrap the bitter end around the standing part and underneath itself twice more to ensure the knot is secure, pulling the bitter end each time to draw the knot tight.
The "Fisherman's Bend" is also known as the "Anchor Bend." If you need to untie the knot, simply insert a long pointy object, such as an icepick or marlinspike in the last turn of the bitter end and worry the bitter end free.