Disks configured to float on the surface of a body of liquid, including a top member configured to float substantially above the surface of the body of liquid, a bottom member spaced from the top member and configured to float substantially below the surface of the body of liquid, a sidewall extending between the top member and the bottom member along the periphery of the top member and along the periphery of the bottom member, wherein the top member, the bottom member, and the sidewall collectively define a cavity, a chamber mounted within the cavity, the chamber enclosing a predetermined volume of a gas, and a port defined in the sidewall to allow liquid from the body of liquid to enter the cavity, wherein the predetermined volume of gas enclosed in the chamber is selected to impart a buoyancy force sufficient to maintain the disk afloat on the body of liquid with the bottom member a predetermined depth below the member of the body of liquid.