Clownfish

Clownfish is a simple dashboard widget which displays a video of a coral reef with clownfish and other sea creatures swimming around.

The widget will remain in your dashboard or if you do not like it anymore, you can simply delete it with the click of a button. The widget can be moved across your dashboard the way you want, but you won't be able to adjust its size or customize anything in it.
Sadly, the quality of the video shown is not very good and the sounds seem not to synchronize with the video. Another disadvantage is that you will hear the sounds even if the dashboard is not opened, so you probably should mute them, by pressing the small Pause button at the bottom of the application. This will not pause the video, but will only mute the sounds. However, the video is still nice to watch, so if you like underwater life, you are probably going to like it, despite the quality is not the best.
In short, if you wish to decorate your dashboard with a nice widget showing the underwater life, Clownfish may work for you.