Early Late Cretaceous Sea surface temperatures (SST) records suggest extremely warm Southern Hemisphere high latitudes (SHLs) and a meridional gradient as low as 5°C, attributed to elevated atmospheric CO2. Climate models have been unable to reproduce such extreme warmth, questioning model performance and/or the validity of SSTs reconstructions. Indeed, the latter partly rely on the measurement of oxygen isotopic composition of marine organisms (δ18Oc), a proxy that requires knowledge of the δ18O of past seawater (δ18Osw). Here we use the water isotope-enabled Community Earth System Model (iCESM1.2) to investigate how paleogeography and pCO2 affect δ18Osw distribution and our understanding of Cenomanian-Turonian SSTs. Our simulations suggest that the semi-isolation of southern South Atlantic-Indian Ocean resulted in locally highly depleted δ18Osw explaining low δ18Oc measured on planktonic foraminifera. Accounting for this δ18Osw spatio-temporal variability increases the estimated meridional temperature gradient by 5°C and narrows the gap between model and proxy-based reconstructions.