Submarines are referred to as boats rather than ships due to naval tradition. As a former naval officer I can only say that technically losing any vessel could

Submarines are referred to as boats rather than ships due to naval tradition. As a former naval officer I can only say that technically losing any vessel could be the one that loses a war, just as any soldier lost could be the straw that breaks the camel's back. But if your navy is so rickety that the loss of a single vessel is enough to lose then the main deficiency was in planning rather than any specific warship loss. Losing one in peacetime should never happen but is not unheard of even in modern times. See eg the Kursk or the Fitzgerald.