How much of the Earth's surface was covered with water during the Mesozoic?
At that time there were large areas of shallow seas, so small differences in elevation could lead to large differences in water area. I doubt that the elevations are well enough known to estimate the area precisely.
https://dinosaurpictures.org/ancient-earth/#90
The coefficient of expansion of water is only $207 \times 10^{-6}/K$. 15 degrees would give 0.3%.