You made it sound like "surface water" is literally just counting water close to the surface of the body of water, but it's basically any lake/river, so still impressive.

> Lake Baikal is the world's largest freshwater lake by volume, containing 22–23% of the world's fresh surface water, more than all of the North American Great Lakes combined. It is also the world's deepest lake, with a maximum depth of 1,642 metres (5,387 feet; 898 fathoms), and the world's oldest lake, at 25–30 million years.