Returning to sea duty, Underwood was assigned to the Staff of Commander Submarine Squadron Ten as Squadron Engineer supporting submarines on war patrol. In January 1944, he was assigned as Commanding Officer of . On this ship during three war patrols he was credited with destruction of 76,000 tons of enemy shipping, including the aircraft carrier "Shinyo". For each of his war patrols he was awarded a Navy Cross and in recognition of the great successes of the first two patrols "Spadefish" was awarded the Presidential Unit Citation.