An island is defined as a body of land completely surrounded by water, and a lake is defined as a body of water completely surrounded by land. Now consider a spherical planet where the northern hemisphere is entirely land and the southern hemisphere is entirely water. Would you call the northern hemisphere an island, or would you call the southern hemisphere a lake?

I'd call the northern hemisphere a continent, and the southern hemisphere an ocean....

Consider the Earth as it is right now. Start on land. No matter where you go, you're surrounded by water. Start in the water. No matter where you go, you're surrounded by land.

The problem is that they're no third option, so the water and land always surround each other.