This was a tricky one. Puddles are an accumulation of liquid (usually water) that may form by pooling within the depression of a flat surface. By definition, a puddle is generally shallow enough to walk through, and too small to traverse with a boat or a raft. A rally car however is perfect for navigating a puddle.
What worked for me was spotting a puddle whilst driving my rally car, and driving through the puddle (especially when the puddle was the width of the course and unavoidable). The achievement popped when I had driven through thirty or so puddles. To be honest I wasn't consciously working on this achievement and it was a lovely surprise when it happened. Sometimes racing game achievements don't involve a soul-destroying six-month grind, and this is one of them. Which is nice.
If you're an impatient kind of achievement hunter, just run through a bunch of the rallies in Kenya. I think there are more puddles there than at the other locations. Did you know that Kenya experiences an average rainfall of around 600mm annually? I assumed it would be quite an arid place but there you are. Anyways, you've got this so get out there and make me proud.