In the framework of the EMRP project “ENV 58. Metrology for essential climate variables”, a new technique to perform traceable temperature measurements of the sea water column and sea water surface was designed and it is being studied and applied to a real situation in the submarine observatory (www.obsea.com). This new technique is based on optical fiber Bragg gratings, which implies the distribution of temperature sensors along the fiber. In the design of these distributed thermometers special attention was paid to the involved materials in order to avoid the damages of such corrosive environment. These fibers were calibrated as optical instruments and as thermometers, in order to get traceable measurements, as well as, reliable uncertainty calculation of the seawater temperature profile and of the sea water surface temperature, that are being continuously measured. Besides, these new devices are continuously compared to the current thermometers, CTD, located in the submarine observatory