Fragments of ancient seafloor rocks can serve as proxies for the delta 18-O of seawater. Alteration of the seafloor by seawater imparts a very characteristic oxygen isotope fingerprint on the seafloor. The pillow basalt section becomes enriched in 18-O in proportion to the amount of secondary, high-18-O, low temperature mineral growth. In contrast, the sheeted dykes and gabbros generally are depleted of 18-O reflecting the modal proportion of secondary, low-18-O, chlorite and epidote. These trends simply reflect the high water to rock ratio of the seafloor alteration and the temperature dependence of the 18-O partitioning between minerals and water. Superposition of high delta 18-O pillows with low delta 18-O dykes and gabbros is unique and is easily recognized in well preserved ophiolites and has been described from numerous locations. Older, dismembered and highly metamorphosed segments of the oceanic crust may still retain the original seawater imprint because their subsequent obduction and metamorphism was relatively closed to external fluids. Even suites of diamond bearing eclogites from kimberlites still have contrasting high and low delta 18-O eclogites proving that subduction into the mantle is not sufficient to erase the seawater fingerprint. Inspection of all the seafloor, ophiolite and eclogite data reveals no secular trend in delta 18-O indicating that the delta 18-O of seawater has not changed with geological age. Since the delta 18-O of seawater itself is fixed by seafloor/seawater exchange, the constancy of delta 18-O of seawater implies that the scale and style of seawater/seafloor interactions has not changed over time.