I would venture that seaweed or algae is more likely.

I don't think so, if only because seaweed isn't very calorie dense - 4 calories per 2 tablespoon according to Wikipedia (well, the google card that gets all its info from Wikipedia at least).

I think it's much more plausible that we'll go the route Asimov predicted in "Caves of Steel" - genetically modified and processed yeasts, which are much more calorie dense than seaweed, and much better suited to the sort of industrial process that feeding 10+ billion people requires.

As a Korean, I'm glad we're prepared. Jellyfish salad and seaweed soup, anyone? :)

I specifically cite jellyfish, because they're one of the few creatures that will thrive in the increasingly acidic oceans. As so many other species — including those we currently enjoy eating to the brink of extinction — die off, jellyfish will abound.