Severity-Based Assessment of Maritime Accidents in Indonesian Inland Waterway and Offshore Shipping Environments

Maritime safety outcomes in archipelagic countries are strongly shaped by operational environments, yet systematic comparisons between river/inland and open-sea/coastal settings remain limited for Indonesia. This study examines accident frequency, vessel involvement, spatial concentration, and severity between river/inland (RIV) and open-sea/coastal (SEA) zones using national ship-accident records from Indonesia's National Transportation Safety Committee (KNKT) for 2018-2021 (n = 1,523; RIV = 216, SEA = 1,307), with lake-location records excluded from the binary classification framework. The 2021 data represent a partial-year observation; a sensitivity analysis confirmed directional consistency of all findings. Results indicate that SEA accounts for a substantially higher number of accidents, largely driven by fishing vessels in major sea lanes. However, RIV accidents exhibit a higher proportion of severe outcomes — defined as at least one fatality or missing person (44.4 vs. 31.6) — whereas SEA accidents generate a greater mean number of casualties per event (2.29 vs. 1.14), reflecting higher vessel capacity and offshore exposure conditions. Spatial concentration analysis reveals distinct risk profiles: fishing vessels dominate offshore accident clusters across the Java Sea, South China Sea, and Makassar Strait, while tug-barge operations and passenger speedboats concentrate risk on the Mahakam and Musi rivers. Multivariable logistic regression confirms that, after controlling for vessel category and accident mode, SEA accidents have significantly lower odds of severe outcomes (adjusted OR 0.48; 95% CI 0.34-0.68; p < 0.001). These findings indicate that accident severity is environment-dependent and provide evidence supporting zone-specific maritime safety interventions rather than uniform regulatory approaches.