Visitors to Amsterdam will soon notice that behind the Central Station, there are a few very (sometimes extremely) busy ferry connections to Amsterdam North and back.
The reason is there are just a few bridges to cross the water in Northenr direction, and the tunnels are for cars only. 'Slow traffic' is forbidden to make use of the tunnels (pedestrians, bicycles and mopeds).
There are not only ferries to connect Amsterdam to Amsterdam North, in fact the whole part of the Province of Noord-Holland is cut off from Amsterdam by waterways: the IJ and the Noordzeekanaal, that runs all the way from Amsterdam to the coast town of IJmuiden.
In Amsterdam East there is one bridge that slow traffic can use to travel North, the Schellingwouderbrug.
Just next to this bridge are the Oranjesluizen (locks to let ships go through).
Then, if you go West, the first opportunity for slow traffic to cross the water is in IJmuiden - the ship locks there can be traversed.
In between, slow traffic can not go to the other side of the water.
This is why a number of ferries has been set up, that go 365 days a year, and some day and night.
The ferry Buiksloterwegveer is the day-and-night ferry for Amsterdammers and Amsterdam North dwellers. The other ferries that cross the IJ shuttle 365 days a year, but not during the night (though this is shifting too: see the Nachtelijk Westveer.
Further West there are 3 more ferries that cross the Noordzeekanaal 365 days per year, 24/7. They are the Hempont, the Buitenhuizerpont and the Velserpont.
Because slow traffic has no option to use the tunnels, the ferries are all for free, and 4 of them go day and night every day of the year.