 FACT: Towards the end of the film, when the Grayfish is lying on the  bottom, the captain urges the radioman to keep trying to contact the  Bluefin (the other submarine) on the radio, and the radioman  replies, "Im broadcasting, sir," while he works his Morse Code key.  The Bluefin eventually answers. Conventional radio signals will not  penetrate underwater. Even today, submarines are required to surface  an antenna to receive or send operational communications. Extremely  Low Frequency (ELF) radio waves will penetrate underwater, but they  can carry very limited information, and were not in use in World War  II.