Underwater screensavers are usually my favourite because they're the most interesting. There's always something happening and the colours are awesome. Since I've managed to kill every real fish I've ever owned too, it's also a lot less hassle.
Marine Life 3D Screensaver is one of the most realistic I've seen yet featuring an amazing array of fish and coral. You get clownfish, dancing seahorses and many other exotic creatures I have no idea existed swimming in crystal clear water. It's really relaxing just watching them swim around as they seem to have a very distinct life of their own. There are even sound effects although you might want to turn these off if you're working in an office.
For colours, range of fish and sheet entertainment, this is one of the best 3D underwater screensavers out there