Several years ago, we transplanted some watercress from our "lower 40" and put it in our goldfish pond and waterfalls. These pictures were taken a month ago and show the lower waterfall on the left with the watercress in the center. The upper waterfall, coming out of the filter, is on the right.
These plants do so well that we have to continuously pull it to allow the water to pass quickly enough to avoid overflowing the banks of the stream we made. Its' roots filter the water as it returns to the pond.
Our original intent was to use it as a decorative planting. It has turned out to be additives to our salad, our sandwiches and even a soup of its own.
Later in the spring it has pretty white flowers, but try to keep it from going to seed. It can be prolific.
If you have running water in your water feature, consider watercress. One comment of concern, be prepared to keep it cut back, eat it or thin it. It grows well!!!