What is the surface water to land ratio in minecraft?

This depends entirely on the random seed used to generate the world.

Update: This blog post by Notch on Terrain generation in Minecraft may be of interest to you even though it doesn't give you the figures you are looking for. Basically: The terrain is generated from Perlin noise with some manipulations to ensure that there is enough land and water. This means the ratio will fall within some range (but I can't say what it is, sorry).