You plan arriving at the Congress via freighter ship? :p
Since it is mostly a middle-European event with climate-conscious attendees, most will arrive by train if using any sort of public transport. DBs connections to Hamburg, even from Berlin are abysmal if you compare to Leipzig.
I'll be going to Hamburg soon, as it happens, and it really disappoints me that there is no ferry service from anywhere in Britain to Hamburg. In fact, there are no ferry routes between the UK and Germany at all; the only one going vaguely the right direction arrives in the Netherlands.
Getting from Britain to mainland Europe by sea isn't cheap: ferries from Plymouth to Roscoff are usually over €200 each direction, no meal, no cabin, no car - one adult. And the ships are so massive that it cannot be for anything but artificial scarcity; I'd estimate you could fit about ten thousand people on one of their ships before it would begin to feel crowded.
An affordable ferry with a cabin would allow me to set off at noon, cross Britain from west to east, spend the night crossing the North Sea and arrive refreshed the next day in Bremen/Hamburg ready to use a Deutschlandticket or similar to continue by rail within Germany.
What I have chosen to do in the absence of such a service is purchase a €200 Interrail ticket for 4 days, reserve the Eurostar (Channel Tunnel) train for €60, and reserve an extra night at a hotel in Belgium for each direction. End result: a total of more than €500 and 36 hours travelling.
It's not easy being a climate-conscious traveller.
I've recently looked for ship connections. Next to being surprised that Hamburg had indeed no decent ferry services, it was quite difficult to search for and I had to even contact (via forms) multiple shipping companies.
Maybe someone would like to build a climate-conscious travel search engine that includes all these routes.
FWIW the last ferry service connecting Hamburg and Harwich closed down in 2002. As to traveling by container ship, as recently as 2019 I saw Hamburg Süd (now belonging to Maersk) advertising classic passages, but I'm guessing those go to China and southeast Asia considering container freight to/from UK would fit on small feeder ships today ;) If you're into the nostalgic aspect of traveling via ships, be sure to visit MS Cap San Diego in Hamburg, a freight ship (now a still sea-worthy museum) from just before the container boom in the 60s, it's absolutely fantastic and highly recommended.
I did not only mean from Berlin but from many other cities, particularly other big centres like Munich, Prague, Vienna, Frankfurt. Even Zurich has a direct train to Leipzig. Leipzig has historically been at an excellent location within Europe and still is.
Last time I checked any late evening connections Berlin - Hamburg were quite scarce. Maybe they upped it a bit then. Still, Hamburg being in the far north is not exactly great for travel.
Accessing Leipzig from the major population centers in the Northwest of Germany is a major annoyance, though. The train connections to Leipzig basically only work for people in the southeast, bavaria or berlin. For everyone else, Hamburg is much more convenient to reach.
Since it is mostly a middle-European event with climate-conscious attendees, most will arrive by train if using any sort of public transport. DBs connections to Hamburg, even from Berlin are abysmal if you compare to Leipzig.