The impacts of explosive volcanic eruptions on marine operations are poorly documented in the scientific literature and, as a consequence, warning protocols and general awareness of the hazard among mariners are lacking. The purpose of this presentation is to collate information about the effects of recent global volcanic activity on marine vessels and apply lessons learned during these events to improve maritime warning procedures. The worldwide risk to ocean-going vessel traffic due to volcanic activity is not well quantified, and certainly less than the daily risk to aviation and people on the ground. However, in Alaska alone, more than 300 large commercial ships travel great circle routes through and near the Aleutian chain every month. Thus, the potential for volcanic impacts is not negligible in the United States. In the wake of recent volcanic events, AVO, USGS, NOAA, and others are reviewing protocols and mechanisms in place to issue timely guidance to the maritime community concerning volcanic activity. These groups have also worked to develop standard marine warning messages and to conduct outreach to the maritime community on the nature of volcanic hazards at sea. The primary concern is ash fall, which can diminish visibility, interfere with onboard electronics, radios, and possibly navigation equipment, as well as foul air and water intakes and cargo holds. Ash is heavy, especially when wet, and in extreme events could pose a loading hazard. Fine ash is also an eye and respiratory irritant for the crew. Other volcanic phenomena such as volcanic gas clouds, floating rafts of pumice, and tsunami generated by a variety of flowage processes into the sea are also of potential concern for boats very near the site of eruption. Even more dangerous are pyroclastic flows and surges that can be devastating to boats caught in their path, or a sudden submarine eruption directly beneath a ship. These dramatic events, while extremely rare, have occurred historically. Further documentation of eruption impacts on marine vessel traffic in the North Pacific will improve our understanding of volcano hazards at sea.