They only sell day tickets on the canal boat, and it was the last boat of the day, just going back to the ferry terminal near the central station. But it was our last chance so we bought tickets anyway.
The boatman told us not to get off with the other passengers. He took just the two of us right back to our apartment on Prinzengracht, the long way round. He put the bow of the boat against the canal wall and we scrambled off. It was a nice way to finish a great holiday.