"The same ship is at port."
Translation:La stessa nave è nel porto.
"In port" is the common English idiom. It is a contraction of "in the port", but it is a bit more than just a location. It also indicates the ship's current status. Interestingly, the opposite status is "at sea". Anyone who knows about maritime transport will prefer "in port". Anyone who doesn't may well use a longer phrase - or, like Duolingo, a wrong one.
A related status is "in dock", which is like in port but implies that the boat is being un/loaded or repaired. "In dock" is sometimes applied as an idiom to other equipment being repaired: "my car (or computer, etc.) is in dock". Britain's naval history has given English a very large number of idioms and sayings like this. See for example http://www.plymouth.gov.uk/homepage/leisureandtourism/libraries/history/navalhistory/navalsayings/navalsayingsac.htm