Why has the Earth's average sea level risen 'only' by nine inches after one degree C increase in temperature? I am only doing gross calculations here, but...
If the Earth has warmed by about one degree Celsius in the last century or so, and the thermal expansion coefficient of water is about .0002085, then the (admittedly salty) ocean should have expanded in volume by about .02%, or 1 part in 5000.  Right?
Well, the average depth of the ocean is about 10,000 feet, and if the water has (mostly) nowhere to go but up, shouldn't the seas have risen by about 2 feet, not .75 feet?
Ummm, I just thought of something...  Has the ocean not warmed as much as the atmosphere?
You should take into account isostasy. As there's more water in the oceans, the crust underneath bends under the weight, creating more room for it. At the same time, continental crust heaves above the oceanic, hence the small value.
A question here to be careful about is where the location of the measuring point, as it very well may not be stable.