I'm making the Earth a smooth sphere, with all surfaces the same elevation. The Earth radius/diameter is that which is the current sea level. I'm then distributing the volume of the oceans over this sphere. If my calculation were correct, then you would have a uniform water depth of about 2560 meters.

In other words, perfectly smooth continents and ocean basins, all at the same elevation, covered by 2560 meters of water.

In this approximation, you could then add mountains no higher that 2560 meters, if the water was to reach the mountain tops. 2560 meters is essentially nothing relative to the Earths radius - The solid surface is still essentially smooth.

Moose

*Edited by Minnemooseus, : Add 3rd paragraph.*