In 1940 the British state formally protested to the German government about a recent string of attacks violating the neutrality of hospital ships. Ensuing arguments in Britain about effective ways of preserving hospital ship safety from acts of enemy violence also broadened out to include discussions of how to ensure the ‘best possible protection’ of medical staff serving in British warships during the Second World War. Examining the sea as a significant yet neglected humanitarian space in scholarship of the international laws of war, this article critically assesses how wartime British political, martial and medical cultures conceptualised, guarded, and manipulated medical neutrality afloat. As this article demonstrates, enemy attacks on British maritime medical care fed usefully into state production of ‘reassurance’ propaganda which positioned Britain as inherently more compliant with the laws of war than Axis nations. Ultimately, this article establishes that official establishment discussions about safeguarding medical neutrality in Britain’s Second World War at sea played an important role in wider processes of national self-fashioning and debates about what it meant to be ‘British’ in wartime.