If you come up with enough force (if you blow your ballast you will surface quicker than usual) you could cause damage to the DD above. If you managed to "drag" your bow down the keel like a knife you could flood multiple compartments... causing the DD to sink. Saying that though, I have no idea how thick the steel was on those boats. It may just be SHIII's rudimentary damage modeling that allowed you to take her down.
Saying that though, U-Boat hulls weren't particularly strong so in real life I'd say its plausible but not without suffering pretty serious damage to the U-Boat... like damage to the pressure hull or something?
You should paint sharks teeth on the bow of your boat lol *que Jaws music!*
