Both sides should recognize that the process of combining data from an evolving mixture of technologies for measuring ocean temperature into a composite global record is an inherently uncertain process. (judithcurry.com)
Also, researchers started measuring ocean temperatures around the same time that they started setting up weather stations. (globalwarmingsolved.com)
Josh Willis writes a good overview of the challenges of measuring ocean temperatures in Is It Me, or Did the Oceans Cool? (skepticalscience.com)