LotsaWater for Mac is a free water-themed screensaver that looks quite good on newer Macs, even those with retina displays. The screensaver is exactly as it sounds, a rippling water effect that takes over your desktop when the timer goes off for your screensaver. It looks fantastic and it offers a nice alternative to the screensavers built into the OS X system.
To install the file, just open the image and install the screensaver file that is unzipped when you do. Your screensaver and wallpaper preferences will open immediately and you can choose this new screensaver as your default, as well as change basic settings for things like the time it will appear, how long it will play, and any special effects that play in the background. The resulting screensaver looks great and is compatible with newer MacBooks and iMacs, too, so it's a good upgrade for any of the existing screensavers you have.
If the time has come for a new screensaver, consider LotsaWater. It's free, it installs easily, and it looks good on your desktop, quite literally converting the desktop into a wash of ripples in a fantastic visual effect.