The Aquazone series of screensavers are some of the most realistic on Mac and Aquazone Blue Planet doesn't disappoint as a brilliantly realistic aquarium screensaver.
Alternatively, you could just use it for your own enjoyment rather than a screensaver. Aquazone Blue Planet turns your Mac into a virtual aquarium that is rich in marine species, with more than 100 different creatures, including fish, sharks, turtles and jellyfish to choose from. Amongst some of the exotic species on offer are Blacktip Shark, Sea Turtles, Ocean Sunfish, Common Clown, Angelfish and Koi Fish.
You can personalize most of the fish tank backdrops with the species you want and even change the scenarios and scenery. The photo realism of the Aquazone Blue Planet is what really impresses but if that's not enough for you, you can even add your own images to the background scenery. The only slight problems you might find are that some Aquazone Blue Planet tanks are not big enough for certain species of fish (and you're warned when selecting them when this is the case) and also, there program does hang for a few seconds when you change the preferences to add new fish or change the tank.
For stunning aquarium realism with a huge range of fish and your own customizable scenery, you can't do much better than Aquazone Blue Planet.