One thing you could try doing is finding the shiping chanle for the port. Most ports in reality have a deep chanle cut into them so the larger cargo and naval ships can get in and out. I don't know if they are modled into the game or not though.
A word of advice from my two failed attempts at port attacks: Mark your path in so you can falow it out. Ports are beset with mines and anti-sub and torpido nets. So if by chance, skill or luck you make it in and make an attack you will need to avoid them on your way out and you know that your way in was cleer of them so you might as well use that.
Also while you are right that you don't need to be close to hit a target that is moored at port it is a good idea to get in to at least 1500m range as it minimizes the chance of your torpido striking any nets.
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