[Legbreaker]

And don't forget you don't have to completely destroy the docks to make them essentially useless. Hitting a nearby transportation hub will result in ships still being able to tie up, but not being able to load or unload cargo and passengers all that efficiently. If the rail yards were wiped out, everything would have to come by truck or animal, skirting the possibly radioactive areas. Rubble from the burst(s) is likely to interfere with that also, blocking roads, canals, footpaths and requiring a major effort to clear.
One only has to look at the results of WWII bombing raids to see what might be faced.

Rubble from dockside warehouses could have been blown into the water, making the waterways too shallow for much beyond a canoe. Without heavy machinery, dredges and time, there's very little that could be done to make them usable again even if the other infrastructure is still good.

Sunken ships have also been mentioned previously. To me there seems a reasonable chance saboteurs could have had an impact either before the nukes, or after to ensure the port was out of commission. A couple of limpet mines detonated as the ship is passing though a choke point and the whole port can be closed.