Wire or Netting Cover
Garden centers that sell and service water gardens will sell a black poly netting that can be rigged on to a frame and set to cover the water garden or small pond. You will have to design the frame so that it covers only clear water and does not bother the garden plants. Thin wire or fishing line stretched back and forth across the pond several times (No more than 4" apart) can also deter these pesky birds.
Sometimes herons become pests at fishponds. There are no simple solutions to this problem. However, herons prefer to wade or find a perch from which to hunt fish. You might try placing string fences around pond edges to prevent herons from entering.