How different is the proportion of heavy water ($\ce{^2H2O}$) in the different oceans?
For detailed information, see Isotopes of the Earth's Hydrosphere, especially chapter 3 which is "Isotopic Composition of Ocean Water".

At the surface, deuterium fraction is maximized in regions of high evaporation, and minimized in areas with ice.   

In the deep ocean, there is about 2% above average deuterium in arctic, and 2% below average near Antarctica.  There is a general increasing trend from south to north.