I think it's an intended feature in GWX that you can't evade destroyers if their crew's level is veteran or elite... It's part of the game... A lot of U-Boots were sunk during WWII...
Unfortunately, this game is a so poor simulation that you actually can't use most of the real world tactics to evade them.
But some special mods add a few new possibilities even if some of them are buggy (thermal layers, depth charges noise, ASDIC not working at shallow depths...). Look for Stiebler and H.Sie patches...