UPDATE: Replaced figure.
Using the simple moisture boundary layer model with the range of salt to fresh freezing point temperatures and assuming that sea surface temperature would maintain a relative constant average, the drawing above is an approximate range of equilibrium states. Below each reference layer is a temperature range in degrees K or C and at the bottom, the equilibrium temperature estimate for the deepest oceans, the abysmal depths.
The Saltwater baseline has a lower abysmal depth temperature of 2.73C and the Fresh has a higher 4.39C temperature for equilibrium. True equilibrium is unlikely in a complex dynamic system, so the values are limits not set point. The Moisture Boundary Layer (MBL) variation is 0.95C above and below the average and the Radiant Boundary Layer is 1.98C to 2.06C above and below the average temperature. These variations do not indicate what the "Average" global surface temperature range may be because the MBL and RBL envelopes expand and contract with changes in the available energy. For this example there could be no change in the average sea surface temperature just variations in the area of open water.
While I make no claim that the assumption of a constant average SST is valid, there is reason to believe that the range would be much smaller than the range of any other reference. The abysmal depths are likely the most stable reference and it agrees fairly well with the model assumptions, but there is not a great deal of data on the ocean depths. There is plenty of room for tinkering, but the results resemble reality thus far.