Human error is a dominant factor in marine operations, on a daily basis many accidents occur due to human error during maintenance activities of ships. Assessment of the likelihood of human error is essential to minimize accidents and incidents during maintenance operations of marine engines. Most of the current techniques to estimate Human Error Probability (HEP) require significant technical and manpower (expert) resources. The main aim of this study is to develop a monograph for assessing the likelihood of human error in marine operations that can be applied for instant decision making. Due to the lack of human error data for marine operations, the Success Likelihood Index Method (SLIM) is used to estimate HEP. The developed monograph can be helpful for chief engineers or captains in the decision making process for various scheduled and unscheduled maintenance operations. This monograph could also be used as guidance for ship owners, operators, masters and classification societies to better prepare, prioritise and sort maintenance activities for safe and reliable marine operations. It can serve as a helpful tool to reduce the potential of accident occurrence.