Judy, is arugula available? To my taste watercress and arugula are
similar. If I didn't have either, I'd probably just chop up a bit of
parsley (but not cilantro--its flavor might be too dominant).
Another option: E. Schneider in Uncommon Fruits and Vegetables suggests
using radicchio cut into thin strips as a substitute for watercress in one
of her recipes.