Radar and radar-controlled gunnery are the wartime innovations

What was the most underrated naval innovation of the Second World War?

Radar and radar-controlled gunnery; escort carriers were a major wartime expansion of an older idea, and angled decks belonged to the jet era.

Escort carriers had precedents in earlier converted liners such as Argus, which had already demonstrated the underlying concept. Wartime production expanded and normalized the idea on a far larger scale. Radar and radar-controlled gunnery receive the stronger claim to innovation. Angled flight decks belong later because they become necessary with jet operations, not with the piston aircraft that dominate the war.