I highly recommend using a bicycle when travelling the industrial parts of the harbor. You can get here through the “Alter Elbtunnel” at Landungsbrücken, and I’ve never done this by foot as it might take way more than an hour to reach this spot. However, it is totally worth it, as you have an incredible view to the city and can see many different areas with and without water and with industrial charm. The city of Hamburg has made it part of its “Hafenerlebnisroute” for cyclists.
On the website (only in German…) and on the “harbour experience route” (the literal translation) you’ll find information at the corresponding spots in English as well. There is a 45-km-long tour of the harbour for you if you follow the information tables. But don’t worry: you can do just parts of it as well of course. I love the atmosphere and the tranquility plus the industrial charm (did I already mention its “industrial charm”?). It is a great way to explore hidden city gems for me.