Here's why:

* Vastness: The oceans cover over 70% of the Earth's surface, providing a huge area for life to evolve and thrive. In contrast, land is much smaller.

* Depth: The ocean is incredibly deep, with vast areas that are still largely unexplored. This depth creates different environments and habitats that support a wide variety of creatures.

* Diversity: The ocean contains a huge range of ecosystems, from coral reefs to deep-sea vents, each supporting unique species.

While we know much about the life in the ocean, there are still many undiscovered species, making it impossible to know the exact number.

While land has a lot of biodiversity, it simply can't compete with the sheer volume of life in the oceans.