I have a watercress bed by my back door, in what was going to be a hole to sink a water tank in until I hit a layer of flints. It collects water run off from the flagged area of the garden, and the leaks from the hose, and does not get the sun. I started by sowing some seed I had had success with a couple of years ago, where it wasn't even wet, but it didn't germinate. Then I added two new packets (about 4000 seeds), but they were very slow to grow - a few plants have appeared.
I had success with a pack of organic watercress from the supermarket. Having eaten some in one meal, I put the rest in a glass with water until roots appeared in a couple of days. (It usually goes off, and I hate the waste.) When the roots appeared, I used the dibber to make holes in the damp soil, and dropped the stemms in loosely, like planting leeks, then watered the holes. This has worked well, but I don't have enough yet to use it a lot.