HI, I'm surprised you haven't had more response. Maybe you should go over to the ponds and aquatics forum. Anyway, I have watercress in still water and it is growing like a boeing. It roots very easily, if picking it up at a supermarket get a bunch with some roots.

I've grown watercress in an NFT system for a freind who loves the stuff boiled up with pork bones. It grows like a triffid and the tons of roots quickly fill the nutrient gullies unless you trim the roots regularly.
Commercial growers in NZ grow it on capillary mats by the ton without any problems.
Cheers Rosco

It can quickly get out of control if you do not harvest it regularly. It is also quite attractive to a lot of critters.

I've recently grown somthing called Upland Cress which has a very similar flavor to watercress, but it is a much more well behaved plant, growing in a nice rosette.

Attractive to a lot of critters??? I have it in my pond and it is one of the things the racoons don't touch. Maybe they prefer the water hyacinths, but usually they destroy everything in or near the pond.