Getting out on the water in Germany

In the area of Berlin how about a boat cruise on the Havel river in Potsdam? Much more serene, tranquil and appealing than that on the Spree in Berlin, even though I would not oppose that. It is basically a different feel and experience.

I have yet to do the boat cruise in Hamburg but did that twice in Kiel....very nice and interesting. I don't bother with reviews.

The Maritime Circle in Hamburg is actually wonderful. I've done it numerous times and loved it each time. It's also a HO-HO, so you can get water time in between your stops if that's your thing. However, what locals also do is simply ride the ferries. They are included in your public transport ticket, so on a nice weekend, the ferry out to Finkenwerder, for example, will be full of locals just riding it up and down the river. There is no commentary, but you get great wind and water feel while going past beautiful Blankenese. It's probably the simplest option for getting out on the Elbe. You can also just keep it short and simple--take the ferry from Landungsbrücken to the Elphi and skip the (short) walk. Lots of varied options for length of time on the water and ways to incorporate it into your trip.

I haven't done the Alster boat tours, so I won't comment on those.

Also an option is to rent a canoe or SUP or similar on the outer Alster. You can paddle up the canals of the Alster among the swans or head out into the big lake and take in Hamburg that way. Bobby Reich boat rentals has canoes, sailboats, and rowboats (http://www.bobbyreich.de/), while you can get a SUP at https://alster-surfer.com/ while pedal boats (for up to 5 people) and larger SUPs and canoes can be rented at https://supperclub.de/verleih-pricing/ There are more, but those should get you started. They all have little cafes attached. It's a fabulous way to spend a warm day, and you can get "drive thru" ice cream from your boat at Cafe CaNale while seeing how the wealthy of Hamburg live. Just remember that there is an old law that says you can't insult the swans!

And if you are a hard wind-and-water junkie, there are two further-afield day options I can recommend: A day trip to Heligoland and one to St. Peter-Ording. The latter isn't about getting ON the water, but to the most incredible beach and out in the wind / waves / Wadden Sea landscape. Walking barefoot through the "Watt" is an experience you can't have anywhere else. The former is a boat trip from either Hamburg by catamaran or smaller boat from Cuxhaven or Büsum out into the North Sea to the island of Heligoland. The catamaran trip is awesome--leave from Landungsbrücken in central Hamburg, boat up the Elbe, out across the North Sea, spend the afternoon wandering and bird watching and seal spotting on the island(s), then the same trip back. Full day, but amazing!

I don't do any of this nearly enough, but I absolutely love all of those things.

A day trip to Heligoland

When I read the post, I immediately thought of Heligoland. I think very few American tourists have ever been to 'our' only German offshore island. https://www.youtube.com/watch?v=vfWlpm1Cc9I
Furthermore, Heligoland became known throughout Germany at the end of WWII - the British tried to blow up the island.
I can wholeheartedly recommend everything that's been written about Hamburg and the opportunity to get out on the water there. The harbor tour is great, but there's so much more. I've always liked the Alster boat tour as well.

I've always loved boat tours in Berlin, too.

Maybe not this time, but if you really want to relax, then you can, like Huckleberry Finn, have a great vacation on the water in Germany. The Mecklenburg Lake District consists of more than 1,000 lakes, some of which can be navigated by houseboat, even without a boat license. Zero comfort on a raft with a tiny hut on it, or on a real boat, which offers more comfort. You travel from lake to lake and enjoy life. If you have a license, you could even go to Berlin with one of the real boats. https://www.off-to-mv.com/houseboat
https://www.dw.com/en/a-trip-to-the-mecklenburg-lake-district/video-19401696
And if you're already in this area, the small town of Waren on Germany's largest lake (Müritz Lake) is also worth a visit.

Good and comprehensive advice already.

I just like to add Schwerin, Germany's smallest state capital located between Berlin and Hamburg with a very nice castle and old town. You can rent house boats there as well or do a four-lakes boat tour in widely fresh nature.

If you're deciding to go to Schwerin instead, one other place close by logistically would be worth your time and energy....Ludwigslust, ie, Schloss Ludwigslust. Some years back I was in Schwerin wanting to get back to Berlin, no direct connection on that day.

That route route was done by the bus from Schwerin to Ludwigslust, then the train to Berlin with Berlin Ostbahnhof as the terminus.

That part of Schwerin and Ludwigslust covered by the bus was revealing and interesting relative to German towns.

Schloss Ludwigslust was featured some years back on the 90 cent German stamp for post cards. That 90 cent stamp was the required postage for post cards to be sent to the US. I used plenty of them.