To achieve climate objectives, it is essential to close "energy efficiency gaps"—the discrepancies between potential and actual energy savings. While much research in the maritime sector has focused on policy and onshore influences, it is unclear to what extent research is overlooking the potential of onboard human factors to mitigate climate impact. This study addresses this gap through a systematic literature review of the results sections of 17 journal articles on maritime human factors. Using Thematic Analysis, we generate 12 themes, with the most prominent being stakeholders, knowledge, and technical implementation. These themes provide insights into onshore influences, seafarers' expertise, as well as examples of the usage and limitations of implemented systems. Conversely, underrepresented themes such as learning, system properties, and safety referring to how seafarers acquire knowledge, specific design guidelines for onboard technology, or how to overcome the goal conflict of energy efficiency and safety. Our findings underline that key areas in this field have been studied disparately, and that a complete picture is necessary to close the energy efficiency gap here and in other sectors.