Dream Aquarium works a treat under Linux nowadays. (Though it does not run as a screensaver, you can run it manually whenever you want.)
Install Dream Aquarium through wine – no fiddling necessary.
Download gdiplus.dll from http://www.dreamaquarium.com/download_w2k.html
Drop gdiplus.dll into ~/.wine/drive_c/windows/system32
That’s it. I’ve tested this on a number of systems.
Enjoy the relaxing aquarium.