I'm a fan of SereneScreen Marine Aquarium, although it isn't free. It's just $19.95, though, which isn't bad in my opinion. I don't know if there are better aquarium screensavers out there, but ...

It's with this background in games and digital art that Sachs looked at Microsoft's simple aquarium-themed screensaver for Windows 95 and 98 and thought he could do better.