The acidification of the ware is what goes unnoticed. The heat adds to ocean volume but CO2 acidifies water more but it's not noticeable by humans.
1. As the concentration of CO2 in the atmosphere increases, the partial pressure of CO2 also increases. This tends to increase the concentration of CO2 dissolved in seawater.
2. As the temperature of the seawater increases, the solubility of CO2 decreases.
(1) is much stronger than (2) so far. Atmospheric CO2 concentration has increased much more than CO2 solubility has decreased due to temperature.