The difficulty with this answer, lies in the fact that not all of land and ocean biomes have been completely explored. The ocean is vast, covering approximately 70% of the Earth's surface, with literally vertical miles or kilometers of depth, and with some areas with sparse to no biodiversity. The same can be said about certain areas of large deserts with very low levels of biodversity, void of life, like vast deserts of the Sahara or Gobi. But, the Amazon rain forest contains still unknown species of plant and animal life, just like the ocean. Because of its vastness, intellectually, I would say the ocean contains more biodiversity, but the answer is scientifically, as of now, yet to be proven one way or the other.
