Well, I did the same on an uneventful patrol but snuck into Freetown. It was dark, heavy fog, heavy rain and pre-radar time (11/40) so I felt safe on the surface...NOT!
I was suddenly fired upon from nowhere so I turned around at flank speed, hoping it was a corvette or sth slow. But it was a destroyer hunting me down and ramming me to death. As water was only 11m deep diving was not an option.
Thus ended my successful career after 10 patrols, 48 merchants with 244 000t and 3 warships (1 carrier, 2 DDs)...
I am playing at 94% realism btw.