I would guess that the last time there was more water in the Northern Hemisphere than in the south was about
300 million years ago, prior to the formation of Pangea. The supercontinent of Gondwanaland (modern day South America, Africa, Australia, India, and Antarctica) was entirely south of the equator, and only parts of North America, Europe, and Asia were north of the equator. Further back,
500 million years ago, almost all of Earth's landmasses were in the Southern Hemisphere, with the possible exception of North America and Australia.