I pick and choose because, unlike a real u-boat skipper, I know that SH3 - with or without GWX - is LOADED with juicy targets whose type and GRT do not correlate with reality for the timeline. Given a little advanced knowledge of historical shipping routes during each year of the war, all I need do is go to one of those areas and wait. SH3 is too target-rich for reality. It's like shooting fish in a barrel, until 1941 when the fish start shooting back. Turning off map updates helps level the field a bit.
On one patrol, I sank 3 whale factory ships, 3 ceramic ocean liners, and two troop transports - all sailing alone or in pairs - because I found a spot where GWX routes them in succession along parallel paths.
Realistically, a u-boat was expected to attack every legitimate target it encountered, weather and tactical situation permitting. For all they knew, that might be the only ship they would see on the entire patrol
and every ship sunk, irregardless of size or cargo, reduced the allies' available resources accordingly.