The human factor is regarded as one of the most significant topics for sustainable maritime transportation since increasing shipping activities can pose potential hazards to human life, the environment, and commodity. The paper aims to assess the potential contribution of human errors in the maritime industry. Hence, Success Likelihood Index Method (SLIM) was employed by incorporating interval type-2 fuzzy sets (IT2FSs). While SLIM systematically estimates human error probability (HEP) for the designated task, IT2FS deals with subjectivity in the process of using experts' judgements. A loading operation onboard containership was investigated due to its considerable risks for the marine environment. Safety culture, experience, fatigue, and time limitation were observed as highly effective on crew performance. The obtained results indicate that the IT2FS-SLIM approach can effectively be applied for determining the vulnerabilities and critical human errors in the operational process. The paper is also intended to enhance safety control levels and minimize potential errors in maritime transportation.