As Jim noted, the screensaver does not come with that functionality. But go to
www.versiontracker.com and you'll turn up a number of free programs that let you turn any screensaver into your desktop. (Search "screensaver desktop" -- no quotes -- to see them.) I use "BackLight" with the aquarium, personally, and think it works quite well. It's a great effect.
Be prepared for a somewhat sluggish system, however, unless you have a dual G5... even my iMac G5 slows down noticeably when it's running... this screensaver hits the CPU pretty hard, and it takes a dual processor machine to run well while the background is on.