AudioCubes sells the THANKO MPMOUSE10 USB Optical Liquid Mouse featuring an aquarium with plastic fish.
The optical mouse comes in four flavors: Yellow duck, Green fish), Blue fish) and red clown fish (little nemo right there).
The fish or the duck swim around as the mouse is moved. The unique Aquarium mouse is only $29.00 on AudioCubes.
The Aquarium Mouse was featured at Gizmodo and Engadget recently. See also japanese manufacturer Thanko.