One thing that has gone unsaid so far in discussing "turkey-shoots" and "fish in a barrel" is the actual tonnage scores of U.S. submarines. Compared with the U-boats, the American record seems pretty poor. This is primarily because those four or five unescorted merchants didn't alway follow the same routes and could be pretty hard to find. Also, real submarine captains didn't bravely follow the convoy until they had destroyed it; the usual tactic was fire off all the torpedoes and immediately start looking for an escape route. For one thing, japanese merchants carried depth charges, and while they had on sound gear they could still get lucky on that first pass. And sometimes a convoy would get lucky and find themselves heading in the same direction as a cruiser task force, and so you might have four or five merchants escorted by two light cruisers and six destroyers. Also, there were a lot of smaller escort vessels, and oddly sometimes merchants might be assigned as 'escorts'.
So it wasn't all fun and games; it was just a different kind of war. Find a copy of Silent Hunter-you know, the one that started this series-it's still played by a lot of people and is the basis for the longest-running online submarine campaign: Pacific Thunder.
http://www.subsowespac.org/