Look for watercress growing in cold streams and springs, often growing in dense mats of green floating on the water. The leaves grow off of the main stem in multiples of 3 to 9 small, oval leaflets. The entire plant is succulent and has a pungent smell. Warning: as with wild plant, be sure to correctly identify it, and eat only a small portion the first time in case of food allergies. Pick watercress from waters you know are not polluted. I only gather it from springs near their underground source.
Watercress is best in winter, picking the younger leaves and stems. It becomes more bitter when it blooms in springtime (small white flower with 4 petals, growing on a stalk out of the water). Treat watercress as you would any cooked green. Wash them to remove any grit, and cook them by steaming or boiling. If they are too bitter for your taste, try boiling them in two changes of water. Cook until leaves are tender, similar to cooked mustard greens.
For salads, pick the tender young leaves, wash, and refrigerate until used. Add to a lettuce salad, and enjoy the peppery taste and healthy vitamin content. Watercress is high in vitamins A, C, and iodine. It has been used in the past as a diuretic, blood purifier, used to treat lethargy, rheumatism, heart trouble, bronchitis, scurvy, and goiter. In India a leaf extract is used to correct vitamin deficiency. A good book on wild edible plants is A Field Guide to Edible Wild Plants, by Lee Peterson.