What you've got is this randomized population of "Swimmers" in the "Garden of Eden" (using standard settings, anyway): a bunch of randomly generated things in this puddle, right in the middle of a dense splotch of food. But they eat the food, and it "respawns" farther away. End result? The only swimmers who live- and breed- are the ones that can actually almost swim.
It takes patience. Set it up with whatever looks interesting, then go and leave it. Let it run overnight, or just while doing something else.
Remember at the start of it how badly the population "swam;" perhaps "swim" is too kind a word when "flailed around aimlessly" would do better. Then at time one million ticks (about four hours, on my computer), look at what you've got.
What's interesting is the wide variety of designs that work. There's one really weird three-legged population, mostly green, currently going that does an effective job of "crawling" through the water. There is a much larger population of orange Hyper Sperm. (Anything one-legged that moves that frenetically can only be called Hyper Sperm.) Currently going extinct is the yellow population of Lazy Bastards- they evolved to conserve energy, so they don't mvoe much. Problem is, the Hyper Sperm just ate all the food near them, and they can't move quickly enough to get to food before starving- bye by, Lazy Bastards...
It's a lot of fun to watch. What's really cool is how it's all random- these effective and realistic swimming actions happen because and only because they propogate to the next generation. It's a very good demonstration of evolution.
And lest you worry that you miss it all when you get it going and go to bed, you can make it write a time-lapse file so you can watch the swarms move when you wake up.
Anyway, tomorrow should be interesting- I'll be moving back to campus...