What is realism? Can you see your stealth meter? If you do, dive to 200 meters and complete silence. Or year and date? Does your destroyer have asdic? How much torps you have. If your destroyer has asdic or he just detects you from 200m you shuld just rise to periscope depth and when the destroyer is making her final attack run (when she stops doing patterns and heads towards you (usually when she is about 500m after you), fire your ass torpedo. If you havent got stern torpedoes, use you bow torps.
Rise to 30 meters and try to evade the DCs. When she has passed you, and she is turning for another attack run, fire torpedo.
If you have no torpedos at all, go to deep as possible, silent running, and when you are about 1-16 km away from her (depending from the weather and sky), SURFACE
! When in dark or storm, she can't see you. Move slowly away when she is searcing you from below the surface!