I think your course being off isn't a bug, but rather a realistic feature. Water is never still and waves and the current will continually rock your boat, causing it to veer off course ever so slightly, so your navigator will (automatically) steer the rudder, but there's always the little delay.
Think of it as cycling with heavy sidewinds; you continually have to steer against the wind to maintain course, but it'll never be exact.
Other than that, good luck hunting! I'm sure that with good observations, you'll learn to sink those Tommies left and right!
