Get your watercress from the grocery store - just buy a bundle (no roots necessary) and tuck the sprigs in wherever you want it to grow. Use a rock or some gravel to hold it in place until it gets started.
Here's mine growing in my bog and waterfall about three weeks ago. I pulled most of this out on this day, and then I pulled the same amount out again earlier this week. It grows like crazy. And this actually came back from last year - survived the winter and reappeared in spring. It's great to have in the pond when other plants haven't yet started to grow.
All the green you see around the bird bath stand is watercress: