Have you ever watched the movie Christmas story? The scene where the kid licks the frozen flag pole......
Generally USCG approved whisltes are best, followed by USAF and dog trainer whistles. Ones with NO BALLS in them. They will freeze up full of water or spit.
Fox 40, Acme and Orion make small flat ones. The Jet scream is flat and BIG
Back in the 70's we made squirrel calls out of .223 & .222 casings. we were poor and hungry with no shoes but we had good whistles in our survival kits.