Some years ago, I was doing a little volunteer work with Climate Foundation, and I loved their long-term vision. It was based on these facts: 1) giant kelp is fast-growing brown algae that thrives in cold, nutrient-rich water. It is among the fastest-growing plants in the world 2) kelp thrives when there is abundant sunlight - clear water is much better for kelp than turbid water 3) cold, nutrient-rich seawater is present in oceans worldwide, but in the tropics, for example, it is present only below a depth of about 300 meters 4) kelp needs an anchor-point - it attaches and grows long fronds - it does not grow free-floating.
So the CF vision was to eventually build large kelp farm support arrays at a depth of 30 meters - suspended from buoys at the surface. Cold water would be drawn up from deeper ocean to create a suitable habitat for the kelp at the surface. But ships could still pass right over the platforms, if they could avoid the support buoys. These floating arrays would have the potential to support a vast new fishery in the tropics where pelagic fish are relative scarce in natural conditions. The fast growing kelp would absorb a large amount of carbon dioxide, and kelp fronds that sink in the deep ocean carry their carbon to the abyss where it is fixed for practical purposes for centuries, at least. Finally, vast kelp forests could support a range of industries; tourism certainly, but also a variety of chemical and food products that can be derived directly from kelp or with some added processing.