I've never grown watercress but as far as I know it doesn't have to be in water, just in constantly moist soil. watercress

In what kind of container was the plant growing when you bought it?

In habitat, it grows directly in the stream, but certainly sends topgrowth out of the water as well, so Floral is probably correct.

Unless you can keep it quite cool, it will die or go dormant in hot weather.

We don't know where Sophie is but it doesn't go dormant in Summer here. In fact that's when it grows most. And it's true that in nature it grows in flowing water. But it can be grown in moist soil.

Sorry, I added my location to my name. It's going into winter
where I am, but I think it's supposed to do okay here in the winter
(average winter highs/lows about 17/10°C (62/50°F).
When I bought it, it was just in a small plastic pot, in soil. The nursery had it sitting in a tray of shallow water. I'm thinking that's probably the best way to go...??
I'm also considering building a small pond with a cheap solar pump, as I've got some other plants that could use it too.
Thanks for the link, floral_uk. And thanks everyone for the help! :)

Well both Sydney and most of North America get much hotter than you during the summer. Here it is not usually present in growth after June.

Sophie - I should think it will continue to grow in your Winter. It is evergreen in Winter here and you are warmer. The pot standing in a tray , as at the nursery, seems to be the answer. Now we know where you are located it may well go dormant in the Summer as suggested.

I buy watercress at a Vietnamese grocery store for a dollar and root it very easily in a tray of water. I have kept the tray in a sunny window several weeks with weekly water additions. If you have stinkiness, then I would do some major pruning and keep only the healthy parts of the plant.

It's done great in the pot since I posted this. Spread rapidly, and I got a good first harvest out of it the other day. It's consumed the entire pot, but it's probably become root-bound (I assume because I can't see the soil surface anymore, it's a bed of just roots).
Before I harvested, some areas were turning unhealthy colours (yellow and mottled purple mostly). I'm hoping the prune might help stop that, but it could be a nutrient deficiency.

@chervil2, do you not use any soil then? I used to have it growing without soil at the top of my fish tank, but I read that it grows quicker in soil.

Thought I'd post the happy ending for this, because it's really flourishing after all the messing about at the start.

It's just in a shallow plastic pot, as suggested, and it floats on the
surface of a bowl (25-inch diameter) of water. Haven't had to change the
water since I put it in there about a month ago. We've already had a couple of harvests where I cut it right down to the surface, and it should be due for another soon. Grows like mad. It'll be interesting to see what happens in the summer.

Also just realised that watercress isn't really a herb, is it? I guess the vegetable forum was the place for this.

Good to hear - thanks for sharing! "Herb" has a lot of definitions and by the one I use, it is appropriate here. ;)

FataMorgana