Behind the Amsterdam Central Station, 3 free shuttle ferry services leave for Amsterdam North. Plus there is a ferry more to the West-side of town, and on the East side.
They are all for free and go 365 days of the year. All of them are for 'slow traffic' only: pedestrians, cyclists and mopeds. (Here's why they're free).

Please note: the Buiksloterweg Ferry is the only one that operates day and night. The other two connections to Amsterdam North go until midnight (but still 365 days per year).
The Buiksloterweg ferry is the one you take for the EYE Filminsituut, Tolhuistuin and the Overhoeks area.

All three are accessible for pedestrians, cyclists and mopeds only.

In Amsterdam East you can find the Oostveer (between Java/Knsm-island and A'dam North).