How is ocean heat content measured?
The easiest way this is measured is via Argo floats. They're probes which periodically dive into the sea, and measure the ocean heat content.

This is done by calculating the formula. According to the European Environmental Agency, this formula is:


  Ocean heat content is defined as the integrated temperature change times the density of seawater, times specific heat capacity from the surface down to the deep ocean.


This could also be done manually by calcuating out the formula, and obtaining the temperature of the seawater by using a device such as a Nansen Bottle.