If you look at the capabilities sought for Canadian UAVs, or indeed the ones used by the United States—the Triton UAV that the United States Navy is purchasing—they all carry radar, which can be useful for detecting only periscopes or submarines' raised masts, but they do not have acoustic capabilities, and that's what one really needs at sea. As Commander Sing indicated, that's how you detect something.
On October 18th, 2016. See this statement in context.