This is an odd translation. 'In the port/harbour' is perfectly correct and normal English. 'In port' and 'at the port' are fine, too. 'At port' sounds awkward.
I don't think "The same ship is at harbour." is a good translation. The correct form is "The same ship is in harbour." or "... in the harbour."
A0lso, in English the words "harbour" and "port" do not mean the same. While harbour is just a sheltered place out of the swing of the sea a port is a haven with docks and buildings and all that. And for example in Spanish "puerto" means port, that is, you can only call a man-built haven "puerto" but not a natural harbour. Therefore I think in Italian it must be quite the same, but I'm not sure though.
My several dictionaries, including the unabridged Garzanti, all give both "port" and "harbor/harbour" as meanings of "porto." Some give the optional phrase "porto naturale" to specify a natural harbor/harbour.
"The ship is at harbor" is more likely in US English. I don't think we'd say "The ship is in harbor" in the US.