The biggest thing discovered in the ocean, based on the provided excerpt, is the Jawbone of a giant ichthyosaur from the end of the Triassic period.
While the excerpt only explicitly mentions the jawbone, it implies the existence of a massive creature to which it belonged. More details would be needed to define "biggest" by mass, length, or volume to give a more complete answer. This is also only referencing something discovered and not necessarily still existing in the ocean.
