Doesn't Ceres have a diameter of less than 1000km? Even if it's entirely ice - pi*500km^2 = ~785,000 cubic kilometers of water, right? Isn't that half of what

Doesn't Ceres have a diameter of less than 1000km? Even if it's entirely ice - pi*500km^2 = ~785,000 cubic kilometers of water, right? Isn't that half of what's in the oceans? What am I doing wrong??