Let us assume that the surface area of the earth is 30 square meters (just an assumption so that we can divide easily by 2, 3, 5.)
Since the ratio of land to water on Earth is 1:2, there are 10 sq. meters of land and 20 sq. meters of water on Earth.
Given that there is 10 sq. meters land total and 6 sq. meters in the North, there must be 4 sq. meters of land in the Southern Hemisphere. And the same there are 11 sq. meters of water in the South.
Therefore, the ratio of land to water in the Southern Hemisphere is 4:11.