When in Zeebrugge I am by car, and with a gentleman who cannot walk very far, and we are particularly interested in tugs and smaller vessels. Also I am not familiar with how much flexibility public transport allows (or doesn't allow, taking Pieter's comment). Also it depends on what kind of shipping you are interested in.
I have annotated the attached map with the areas I typically visit. The two viewpoints mentioned by JVCMACGM have arrows (there are no "facilities" at either). The area inside the main lock is good for car carriers etc on the move or at the nearer berths, depending on the time of day, but would be beyond normal walking distance. The current fish dock and small shiprepair berths (lower centre) are also accessible. Otherwise the old fishing dock is now mostly a marina, though some smaller windfarm and survey boats berth on the south side (but almost always have yacht in the way from across the dock).
However, though I have not tried it, the saving grace may well be the Franlis harbour round-trip of 75mins, running from April to October:http://www.franlis.be/port-tour/practical
[later] forgot to mention that for some strange reason there are big gaps in Streetview there are big gaps in the coverage, even around the older residential areas nearer to the port basins