One of the large problems with a subsim is that it generally isn't an action game. To simulate a WW2 submarine you don't find a contact all the time. Personally I don't have a problem with not finding enemies to sink in campaign mode but I can see how somebody new to the genre would find it difficult. This is why the missions exist. Campaign mode was a request from old fans of previous subsims dating back to microprose.
The best tool you have to find enemies is the trade lane map that either came with your edition of SH4 or is available somewhere around these forums. The gameplay of a subsim involves searching for contacts on sonar and radar, many people actually think the campaign layers offer too many targets. Multiple Yamato battleships is a good example.
My advice is to stick with it. Learn the game and you'll either love it or find it boring.
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