This paper introduces a phase change material (PCM) based heat sink (PCMHS) to reduce the idle and operating temperature of a direct current (DC)/DC converter in an underwater battery power system. The developed macro-encapsulated PCM enclosure features a dampening assembly that maximizes thermal contact and eliminates void space catered for volumetric expansion of the PCM during melting. The operating temperature of the DC/DC converter at steady-state (both idle and full-load cycle condition) is estimated via a validated thermal model. A percentage improvement of at least 27% can be achieved by PCMHS at different load cycle conditions tested. The PCM's volume expansion of approximately 6.4% corresponds to the measured cover movement of around 3 mm. The relationship between the state of expansion (or melt state) to solid-state of the PCM is also established.