Hamburg is a comparatively Shipspotter-friendly port. There are literary dozens of good spots.
It all depends on your preferences. A port cruise (Hafenrundfahrt) is always a good choice, if only to rest your legs for an hour and have a pint and still do shipspotting. It's a little overpriced, though.
For in-and-outgoing traffic Bubendeyufer is hard to beat. Take the ferry from Landungsbr