Hello,
if I am going just for a weekend, then I go to Hamburg. Hamburg is easiest, because of a few reasons: It is easy to reach by train. You can use the public transport not only to get TO the port, but a day-ticket includes also the port-ferries. So you can use these to get to various spottiing-points or even go on a photo-shoot, being on one. There are many harbour-excursion boats every day, so if you want to get closer to the ships, pay 10 Euros and go for one hour and it provides a good chance to take photos. Many of my photos are taken at Hamburg, and I will go to Hamburg in the future too. In summer you can also simply stay at the Elbe beach, take a book with you, read and wait what comes by, the day comes and you have a really good and relaxing time, and in the end many photos. Just a warning - if you do not like container-feeders, Hamburg might not be the right place, they have MASSES of these :-)