Regarding this, I noticed in the Travel section lesson 2 in the "Select the picture" questions, η βάρκα's image was of a very large vessel that I would label as a ship and the image for το πλοίο was a rather small vessel that I would call a boat.
Link to the section/lesson: https://www.duolingo.com/skill/el/Travel/2
Anyways, those questions don't have a comment section and the only report option doesn't allow comments so I wasn't sure it would be useful. Hope it was alright posting here in lieu of making a thread.
I don't have a problem with interpretation of the word πλοίο, I have a problem with how the word "boat" is interpreted here. I disagree with your statement of the two words not being synonymous, synonyms are words of the same OR similar meaning. If you think the words are not synonyms, no wonder there's an issue with interpretation.
"Αεροπλανοφόρο" (αεροπλάνο+φέρω,=something that carries airplanes) is the translation of "aircraft carrier", but I guess we'd call it a ship. It's understandable that there are linguistic differences, but "ship"="πλοίο" is the translation we can offer as far as Greek is concerned.
((EDIT)) This comment contained an error. The word removed was "boat". Thank you for the notification. We have removed "ship" and sorry for the confusion. The "hints' are like dictionary definitions so there may be several definitions but not all fit in each sentence. It's usually best to choose the first hint. Here's an example: "Η γυναίκα τρώει στην κουζίνα." under "κουζίνα" you'll see "kitchen" "stove" "cooker" but only "kitchen" fits in this sentence.
Again thanks for your observation.
The point was not to remove "ship" from the translation/hints. Translating the word "πλοίο" as "boat" is perfectly correct. If you want to point out the difference between the Greek words πλοίο and βάρκα, put it in a sentence together, for example "το πλοίο είναι μεγαλύτερο από τη βάρκα"
Greece follows the metric system. Here's an interesting link.