Once a long thin tidal bay open to the sea at its western end, the IJ (pronouced 'Aye') is now little more than a wide river, that connects via its eastern end with the North Sea through the Nordzeekanal.
That said if you stand on Amsterdam's current water front behind the station (the original waterfront having been cruelly cut off by the arrival of the railway) it is still possible to get a whiff of open water.
Maybe it is the sight of the large cruise ships moored up at the Passenger Terminal and the other large ocean going vessels that are occasionally spotted; perhaps it is just that the water is slightly brackish, or it could just be the ghosts of the old Eastindamen (a type of ship) that brought their riches to this place during the Golden Age.
Blue and white GVB passenger ferries operate from behind the station to a number of destinations. There is little reason to want to visit the north shore of the IJ, but they are free and quite fun, if you like that kind of thing.
Most of the canal tours run round the back of the station and on to the IJ, but sadly the Fast Flying Ferry to IJmuiden and the North Sea coast ceased running in 2014.