hallo Russ
there is little traffic nowadays in the centre of Amsterdam, only cruise ships and some drydocking!
having said this though, the Amsterdam port area (including IJmuiden, Zaandam and Beverwijk) is the 4th largest seaport in Europe and the 15th of the world, so plenty of ships coming and going.
to observe them one should go to the banks of the Noordzeekanaal (Northsea Canal)from Ijmuiden to Amsterdam (25km long) there are nice spots to photograph them (even a hill for a "skyfoto")
Ijmuiden locks is the place to watch them all and always interesting ships around!.
also of interest and a nice spot is the Lighthouse in Ijmuiden (south pier) at the harbour entrance where one is a little bit higher up to get some nice shots.
hope this info will help you a little bit further (and everybody else)
kindest regards
chris