Though to be fair the US Navy was actively involved in the Atlantic before Dec 1941.
Either the casualties inflicted on USS Kearny by U-boat U-568 on October 17, 1941, or the sinking of the USS Reuben James by U-552 on October 31, 1941 might be considered the first American naval losses of World War II. The United States was neither officially involved in the war at the time nor did the incidents cause them to declare war.
And of course Brazil in Aug 1942, Argentina in 1944 and Chile, against Japan in 1945.