I get my watercress from a nearby stream in the early spring (which
reminds me, I need to get up there again soon and get some) WC likes
moving water, so the waterfall area of my pond is a good place for it.
It grows quickly, so I have to keep an eye on it to make sure it
isn't clogging up the waterfall and diverting the water elsewhere
outside my system. Also, I've had fair success starting it from seed
and then putting it in the waterfall after it has developed a good
root system. I found seeds at my local garden center (Colorado)
Good luck,
Gary