Mac OS X Hints contained the following hint.

Run a screensaver on your desktop with the command (all on one line)

/System/Library/Frameworks/ScreenSaver.framework/Resources/
ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background

The writer also recommended the Marine Aquarium screensaver. I tried this, and it works, but is too heavy on an older Mac. You need a Quartz Extreme capable Mac and a fast processor to enjoy the experience. But the Marine Aquarium screensaver is a good buy regardless. Have been using it for some time without any problems whatever.