It can, but only as a stem for compounds, especially adjectival:
- „statek pełnomorski” = 'sea-/ocean-going vessel/ship'
- „turystyka morska” = 'maritime tourism'(includes ocean-based tourism too)
- „rejs pełnomorski” = 'sea-/ocean-going cruise'
- „połowy pełnomorskie” = 'Deep-sea fishing'(includes 'ocean deep-sea fishing' too)
- „strefy pełnomorskie” = 'Off-shore zones'(includes 'ocean off-shore zones' too)
Otherwise, „Ocean” is 'ocean' and „morze” is 'sea'.
In American English (and this course is in American English), when speaking generally about a body of salt water, I think you're probably right, the distinction only being made when talking about geographical entities. I hear Americans mainly referring to the ocean rather than the sea, which is perhaps not surprising as they/you are surrounded by the stuff. From the US National Oceanic and Atmospheric Administration:
'Many people use the terms "ocean" and "sea" interchangeably when speaking about the ocean, but there is a difference between the two terms when speaking of geography (the study of the Earth's surface).'
As I read it Duo's sentence is not speaking of geography or referring to any specific geograpical entity, such as the Baltic Sea, but to some unspecified body of water, where Americans might well say "the ocean".
In fact the NOAA themselves. perhaps unwittingly, say "when referring to the ocean" when in BrE we would say "when referring to the sea".
I think perhaps you should accept it. We don't really make this distinction when we're looking at a piece of water.
And there's a difference in usage between AmE and BrE here. While a Brit would say "Look at the sea", even if they were looking at the Atlantic, an American would probably say "Look at the ocean", even looking at the Baltic. Neither of us think "Now is it an ocean or sea I'm looking at, we just use our respective generic word. The difference is only really geographical.
I think you're right for being strict here, Jellei. What we all have to understand is that Polish is NOT English. While it may be true what WarsawWill has said, Polish clearly and strictly wants "sea" as "morze" and "ocean" as "ocean".
Accepting "ocean" for "morze" would be like accepting "puppy" as "pies" (and, as we all should know", "pies" is "dog" and "dog" alone!).