Ocean farming for Biofuel

I was wondering, if the main restraints on Biofuels after peak oil are the lack of space and the lack of oil based products for the farming process, then why not farm Kelp and Algae in the ocean for Biofuel, they grow fast, yeild the same amount of fuel as land based plants, and the ocean takes care of them for us. All we would have to do is plant and harvest it, so what do you think.