Human errors during maintenance operations are one of the most prevalent causes of marine accidents. Seafarers conduct marine system maintenance on-board in a challenging environment, which makes maintenance prone to un-intentional errors. To address this concern, the study of human performance during maintenance operations on ships is necessary as a part of maritime quantitative risk assessment. However, there is a significant lack of appropriate field data and information relating to human performance on-board ships. This study attempts to fill this important data and knowledge gap. It presents a data collection and analysis procedures for maintenance operations of marine systems. Data related to performance-affecting factors is collected from a total of 235 experienced seafarers from Engine Departments (ED) and Deck Departments (DD) through a structured questionnaire. The collected data is then analysed for normality and also for a pairwise significance test. It helps to study the generalization of the data and also to identify the relative importance of the performance-affecting factors. Collected data will help in developing human error assessment techniques for more accurate Human Error Probability (HEP) estimation in marine environmental conditions. Additionally, this study is useful for identifying the relative importance of performance-affecting factors for the maintenance operations of marine systems. Based on the results of this study, workload and stress, and ship motion (roll and pitch) are identified as critical factors affecting seafarers’ performance during maintenance operations. These identified high important performance-affecting factors will assist in future human reliability analysis and risk mitigation strategies for improving the safety and reliability of maintenance operations for the marine industry.