Watercress will grow in almost anything as long as it has water. It will grow is saturated sand, saturated gravel, wet soil, or just in water. I think wet to very damp soil will be fine. If it gets too disgusting, just rinse all the soil away from the roots and plant it in saturated sand or gravel. Watercress likes some nutrients, so use a very diluted solution of miracle grow instead of water. I'm thinking a solution maybe one-fourth to one-eighth of the recommended level for fertilizing houseplants would be good.