here's some specs on the 8.8cm naval gun:
http://www.navweaps.com/Weapons/WNGER_88mm-45_skc35.htm
a 20 pound projectile moving at 2,297 fps is going to do some serious damage. I have a .30-06 that travels about that fast at 200 yards and anything it hits is freakin' mince meat..and you measure its weight in grains!
factor in the HE round, and you've got a hell of a punch for such a little boat. a few hits at or below the waterline and you're history. I guess you could sort of look at he u-boat as a naval sniper in this role. Laying in wait, closing in fast and furious!