Within the land, air, and cyber conflict contexts, the concept of "civilian direct participation in hostilities" has been subject to intense scrutiny, interpretive endeavor, and operational application over the last several decades. There is a risk, however, that interpretations and applications based on this experience may inappropriately permeate the law of naval warfare, where the scope of application of this concept is considerably narrower. This is because a key difference between allocating law of armed conflict status ashore and at sea is that the law of armed conflict status of people at sea tends to follow the status of their vessel. Consequently, the status of a merchant vessel crew is generally a direct reflection of the status and conduct of their vessel. This article therefore seeks to assess the scope for application of direct participation in hostilities within the law of naval warfare context by analyzing what this body of law directs in terms of the conduct of the most likely groups of civilians to be encountered at sea – merchant mariners and, to a lesser extent, passengers. The analysis suggests that under the law of naval warfare, direct participation in hostilities at sea is relatively restricted to active resistance or hostilities perpetrated by passengers, whereas actively resisting or even hostile neutral and enemy merchant vessel crews should at most be made prisoners of war, and should not be treated as civilians directly participating in hostilities.