I don't, the ocean is a big place and we haven't seriously started to farm it anywhere near it's full potential yet (bet we do before long though). Now granted

I don't, the ocean is a big place and we haven't seriously started to farm it anywhere near it's full potential yet (bet we do before long though). Now granted many areas won't be suitable for seaweed cultivation, but if the demand is there, humans can get really innovative. The thing is, we don't make plastic out of seaweed yet and likely won't until we have to, and maybe never at all. I do like your other idea better though if feasible. Save the ocean farming for food.