This paper describes a technique to perform traceable temperature measurements of the seawater column andseawater surface, based on optical ¿ber Bragg gratings. The paper explains the di¿erent phases of the work done:design of the optical ¿bers, its optical and thermal calibration and onsite measurements of the seawater tem-perature. In the design of these thermometers, special attention was paid to the involved materials in order toprevent any damage of the ¿bers due to the exposure to such rugged environment.The ¿bers were subjected to optical and thermal calibration, with the aim to get traceable measurements andreliable uncertainty calculation of the seawater temperature. The ¿bers were deployed in the Mediterranean Seaand water temperatures were continuously monitored and compared with the most common used thermometerin this environment, CTD, located in the submarine observatory OBSEA.