There is provided a marker buoy useful to indicate the location of an underwater structure. The buoy is made from a material having a density less than water, including first and second elongated side members and a central portion therebetween. The central portion receives an elongated twine having a weight attached to its far end. Each side member has a stud extending away therefrom at opposing ends of the buoy. The buoy automatically rotates to dispense the twine when the buoy is placed on the surface of water, and is adapted to be manually rotated by manipulation of the studs to retract the twine.