Marine accident investigators utilize the SHEL model for evidence collections and analysis of causal factors in accordance with IMO resolution. This chapter looks into subclassification of each element of the SHEL with actual marine accidents and statistics that reveal a variety of contributing factors. The accident was spotlighted by media on the master’s impairment from alcohol, but the focuses are fatigue and its implications. Experience and recency of sailing in the water during night or day are often related to accidents. In marine accidents, assumption is distinct from other psychological factors. The Tokyo Bay and the Ise Bay are the most congested waters of marine transportation in the world. Convergence and divergence of marine traffic with numerous vessels occurs at Uraga Suido at the same time on a daily basis. Accidents involving enclosed space entry, where mariners or workers work in different atmospheres, occurred every year.