Did you really get hit? Or are you assuming the torpedo is headed for you? Unfortunately the dynamic missions aren't really realistic, rather arcadish if you ask me. You are dumped into the mission in a too small random area, and the friendlies and foes far too close so they detect each other almost from the start. Then it's over before you know it. The solution is to stay slow and quiet and hope for the best you stay alive, until you get some sensor data on the others. Especially once torpedoes start flying around the others should start to flinch and speed up to avoid.
If you can't get any lines on the sonar display then you may need to adjust the gamma and brightness settings for DW to make them stand out. You can add a gamma setting to the dangerouswaters.ini file. Otherwise adjust brightness settings in your graphicscard driver configuration application.
Code:
[Graphics]
. bla bla
.Gamma "some decimal number between 0 an 1"