All the comments about LotsaWater are all very well, and I agree that that's a great screensaver that's free.
However, Desktop Puddle is better, and it's pretty cheap. I certainly don't regret paying the shareware fee for my copy; it's my favourite watery screensaver.
Some people are saying that LotsaWater is more realistic than this. I disagree; I've just compared the two and find that (a) they're very similar and (b) for me, Desktop Puddle has the edge. Horses for courses, but I wouldn't call it a valid major criticism of either saver; they both look very good, and are very similar. I think that Desktop Puddle's 3D highlighting option for its waves (which LotsaWater doesn't have) helps and make it look a little more realistic.
LotsaWater has a few more options for controlling the detail of the splashes and dimming the screen, but Desktop Puddle has a couple of worthwhile features that LotsaWater doesn't:
(a) the curved screen depth option, which I personally really like;
(b) better image placement choices (current screen, current wallpaper image or random image from specified folder).
Overall I have to say that this is one of my favourite savers, and I like several other of the UselessCreations ones too. Yes, they're shareware, and generally speaking I don't like paying for screensavers, but I've found several of these to be good enough to be worth the modest fee.