3D Desktop Aquarium is an aquarium simulator that can be run as a dynamic wallpaper and/or screensaver. The screen fills up with moving water that distorts the desktop and fills with swimming fish of your choice. Presumedly due to sandboxing restrictions, the screensaver runs external to the System Preferences pane, and the application must therefore be open if it is to run.
Even though the graphics are pixellated and there is a prominent button and placeholder image that advertise the developer's other "useless" offerings, the smooth animation of both dynamic fluid and fish movement, as well as the diversity of fish available in the full version compensate for those shortcomings in my opinion. The aquarium is highly customizable as a bonus, with up to forty fish on the screen at a time in a variety of sizes.
Sam's Protip: It appears that version 1.9 broke several features of the application, such as use of certain backgrounds and distortion of water based on the Mac's motion sensor - cool if they worked right? But not quite as cool as sharks, which version 1.9 added, so I think it's a fair trade.