Offshore electric systems for ocean energies need to be highly reliable in order to avoid prohibitive maintenance and reparation operations. Power electronics is often used to handle the output power from the devices. The first step of the conversion consists usually in rectifying the power into direct current, allowing for connection to a DC-bus. The diode bridges are subjected to severe power fluctuations. The resulting temperature cycles are damageable for the semiconductor modules, and might shorten their lifetime. Submerged devices can take advantage of the surrounding water to be passively cooled. Wave energy converters, equipped with diode rectifiers, have been deployed and tested by Uppsala University on the west coast of Sweden. Experimental current and voltage data along with a circuit thermal model have been used to evaluate the junction temperature. This thermal model was also validated with a laboratory set-up. Results are presented for the junction temperature model and for the validating set-up. This estimation of the magnitude of the temperature cycles will lead to a better understanding of the phenomena limiting the use of power electronics components in these specific operational conditions.