The Deutsches Sielhafenmuseum (German sluice harbour museum) in Carolinensiel is located at the German North Sea coast. The harbour has been restored to its historic state and is home to several historic sailboats. You can take boat trips there and learn about the history of land reclamation and storm floods in the area.
In three museum buildings they show objects of ship building and handicraft, life on board and history of the area.
When we visited Carolinensiel on a sunny day in April 2015, the weather was fine and we very much enjoyed the relaxing atmosphere of the harbour. We had a beautiful day there.
There is more information (in German only) on the homepage of the Deutsches Sielhafenmuseum in Carolinensiel.
I include photos of two of the ship paintings brig Polaris, bark Bertha, and of the model of the schoonergaliot "Anna von Carolinensiel" of 1870 and two displays on the harbour's history.
If you click on a photo you will get that photo in high resolution.