Like any real history, evolution is not driven by a single force or idea. Horizontal gene transfer from bacteria obviates the quest for an “ancestor” seaweed. Maybe there isn’t one.
It’s not “land” vs. “sea” that’s really significant here. It’s how much time was available for the development of photosynthesis. If the claim is that photosynthesis developed via natural selection acting on random mutations (Darwinism), then it must have somehow randomly happened in that billion years. Was there enough time? becomes an unavoidable question.