Run For Your Lives
I haven't heard a reference to the California Giant Kelp for ages, probably since I was in grade school. Back then, the Ripley's Believe It or Not incredibility behind The Kelp had to with its rapid growth. Up to TWO FEET per day. Believe it. Or not. I don't know what I thought back then, but I can see how it can occur. The misguided limitless energy enthusiasts in the referenced link, however, think that fast growth will lead to the possibility of synthetic gas energy cultivated from the monstrous stalks.
Basically, seaweed contains mostly water and the salt water growth medium by itself can support huge structures which have nearly the same density. Otherwise known as the neutral buoyancy effect from Physics 101. We can all think of lots of first hand evidence to support the low organic mass of seaweed. On hot summer days, huge mossy algae blooms of several feet in diameter can seemingly spontaneously appear. Lakeshore lot owners who dredge their shorelines for coontail into huge piles, will see that pile shrink to a fraction of its original size after drying in the sun for a few days. Bottomline, anything that grows that rapidly can't have a lot of meat to it, so to speak.
It would be great if kelp could provide a renewable energy source, but like the personal jet-packs, giant giberrellin pumpkins, and other Popular Science ideas of our youth, innovative ideas are often tantalizingly close but ultimately out of reach of practicality.
UPDATE: Historical account of experimental kelp farming. Apparently, just scratching the surface and you run into problems with the concept.