Could you live off of watercress?
nope. No B12 or D in there and both play many different roles in body growth, cell regeneration, etc. ALSO little to no B vitamins of any kind, iron or protein. AND here is the kicker: you would have to eat roughly 500 cups of cress to get at least 1500 calories. Good luck with that.
Don't think so.
No. It is a good food, but it will not supply all of the vitally-important nutrients you need.
No. It has no B12, of course, very little protein per serving. You need to note serving sizes in nutrition data.
No it's got no protein or carbs in, also the amount of oxolate and anti oxidants would poisen the person as they would have to eat huge amounts to get enough calories
There is NO one single food that you can sustain a long and healthy life by eating it alone. *VARIETY* is the key. Many foods may pack a lot of nutrients, but there's nothing that carries all of the nutrients we need.