alt Hacker NewsWiktionary's note says some people think this root may originally have referred to freshwater/inland seas, not oceans.
https://en.wiktionary.org/wiki/Reconstruction:Proto-Indo-Eur...
In many of the daughter languages it does regularly include saltwater connected to the world ocean (certainly the Latin "Mare Nostrum" for the Mediterranean!); it just seems uncertain from this description whether it necessarily had that sense in PIE.
That doesn't seem like "had no ... word for ... 'sea'" is a very clear or helpful description of the situation, though.
That doesn't mean no word for sea. It means sea refers to any large body of water that you dnn't see the other side of, like in some (all?) other language families. There is no discernible distinction from the perspective of the ancient observer. The Great Lakes look like oceans. Saltiness isn't of great relevance since there are salt lakes, too.