Hamburg as a city is a city with maritime flair. There is the harbour, the Alsterlake, lots of canals throughout the city. The canals are great for canoeing and rowing. On Alsterlake you can rent a small sailing boat and have a day off...
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East of the harbour there are many places for relaxed canoeing and kayaking tours.
Especially one quiet branch of the Elbe, the Dove Elbe, is very popular for watersports. At one part of it, they somtimes have motor boat races. But most of it is relatively narrow and quiet.
Equipment: You can hire kayaks on the Dove Elbe.
Right in the center of Hamburg you find the Alster lake. Here you can hire sailing boats and rowing boats.
Should you want to go sailing, you should keep in mind that the winds are treacherous! Due to the buildings around it, the wind often comes from unexspected directions!
You also can go canoeing through the upper Alster and its tributaries and their connecting canals. This gives lots of possibilities for nice relaxing tours.
Equipment: Boats can be hired at several places.
Although the Elbe has a lot of ocean going commercial vessels goming in and out of Hamburg, it also is possible to enjoy water sports like sailing, kayaking and canoeing.
Of course you have to keep out of the main traffic route! People on those big cargo vessels might just not see you... !!!
So, for a relaxed boat trip it is best to stay just on one side!
Before you go, you also should get information about the tide schedule! Plan your trip according to the tide. Otherwise it might become a little frustrating, when actually the pedestrians on Elbufer-Wanderweg (scenic walk along the Elbe) are faster than you ;-)