I used to think continents and seabed were the same. If you were to add more sea there would be more seabed, less continents. Remove the water and all that's left is a single continent.

But that's not even close. For starters, the continents are ancient. The seabed, the crust between the continents, is in a constant process of renewal. The age of the seabed is measured in millions of years, not billions.