billy, even though the oceans are more accessible than our atmosphere, there may be even greater problems getting an overall temperature that is representative. I'm not sure that scientists even know all the currents that flow at various depths, or are able to monitor the oceans at at depths. Given they estimate that they have only identified a fraction of all undersea volcanoes, their knowledge of the oceans still has a lot of gaps that need to be known. Whilst it may be relatively easy to measure temperatures at given points at set intervals, but unless those points not only cover the whole expanse of the oceans but the entire cross section at regular distances how are they going to be able to determine what volumes of water are at what temperatures in order to come up with some temperature benchmark figure.
