Just divide the volume change by the total ocean surface area, 360 million square km. (tamino.wordpress.com)
With the contribution of such record warmth at year's end and with 10 months of the year record warm for their respective months, including the last 8 (January was second warmest for January and April was third warmest), the average global temperature across land and ocean surface areas for 2015 was 0.90 °C (1.62 °F) above the 20th century average of 13.9 °C (57.0 °F), beating the previous record warmth of 2014 by 0.16 °C (0.29 °F). (ncdc.noaa.gov)
Changes in the evaporation rate from, say, higher or lower humidity in the air above, changes in windiness or changes in ocean surface area caused by waves will have a similar effect in varying the characteristics of that 1 mm cooler layer. (newclimatemodel.com)