Amsterdam Noord is separated from the centre by a large canal called the IJ. The most convenient way of crossing this canal, both during the day and the night, is to take a ferry. Because there are a lot of ferries operating in a relatively short route you rarely have to wait more than a few minutes.
Most of the ferries depart just outside Amsterdam Centraal Station, other places to get on board are displayed below. The best thing about this service is that it is absolutely FREE!