Kelp, only 4 calories per tablespoon
Haha - kelp does have all kinds of medicinal uses, iodine, vitamins, obesity fighter, thyroid regulator and is in Benni's dog food. I've tried chewing on leaves and it basically tastes like salt. Very hard to chew up and swallow in its natural form.
Mostly I like watching it swirl around in the ocean but I have seen kelp being harvested. Giant kelp can grow 1 meter per day and is one of the fastest growing organisms on Earth.