This is a rare case of an HN discussion on international law where there is something approximating an RFC that we can just go consult on these issues --- it's the San Remo Manual, which is trivially Googlable, and consists of a series of numbered paragraphs. Cite the paragraphs that support the argument you're making about the unacceptability of sinking a flagged enemy warship simply because the attacker knows it to be unarmed.