I'm not sure seed is freely available of Water Cress. Why not just buy a packet from the supermarket and root the stems in water. Many have roots on anyway or will root very quickly if not.

It normally grows in gently running water in streams but can be grown in very moist soil. It doesn't however like growing in stagnant water such as many ponds so either grow in a stream if you are lucky enough to have one running through your garden or make a bog garden with liner filled up with top soil to keep moist.