You're right - the 90% under/10% above proportion is true. However, the density of ice is lower than the density of water, so the same mass of H2O will take up a larger volume as ice than as liquid. Basically, any object that has a lower density than water will float in water, and that object, as it floats, will displace a a volume of water that has a mass equal to the mass of the object (Archimedes Principle, IIRC). So, a ship is displacing a chunk of water that has the same mass as the ship - because the ship is less dense than this chunk of water (much of the ship is hollow, after all), it floats partly above the sea surface.
Here's an example with the ice: Assume you have a large pool of water that is all liquid. You mark the water level on the side. Now, take one pound of that water and freeze it into ice - you get a pound of ice that has a lower density and a larger volume than when it was liquid. (In fact, this pound of ice is about 10% larger than the equivalent pound of water.) Put the ice back in the pool, and it floats, displacing a volume of water equivalent to one pound. The water level doesn't change at all - you took out a pound, then put back in an ice berg that displaces one pound's worth of water - but part of the ice floats above the surface because its density is lower.
What happens when the ice melts? As the ice turns into liquid its density decreases, but you still end up with one pound of water, which is the exact amount of water that the ice was displacing. Again, the water line remains the same.
On the global scale, imagine a million ton ice sheet - it's floating on the surface displacing a million tons worth of water. When this ice sheet melts, it adds a million tons of liquid water into the ocean, and sea level remains constant....