To compute the total energy requirements, you don't really need to know the ocean volume, if you are willing to linearly approximate the thermal expansion. If you have less water, the same amount of energy will heat it up to a proportionally higher temperature and correspondingly expand it more, resulting in approximately the same sea level rise.
Obviously this breaks down if you want a more accurate figure, but in that case you also have to account for water shrinking with increasing temperature between 0 °C and 4 °C and so on.
In any case, melting a bunch of ice is a much more energy-efficient way of increasing the volume of liquid water than heating already molten water. The energy required for melting ice is roughly the same as that for heating the water to 80 °C, and 80 °C is not twice as voluminous as 0 °C water.