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, "I'm broadcasting, sir," while he works his Morse Code key. The Bluefin eventually answers. Conventional radio signals will not penetrate underwater.
However, the QC sonar onboard WWII submarines was set up so that it could be used in conjunction with a straight key for Morse Code sonar pulses for emergency communication, so the scene depicted is plausible.