It's quite arbitrary. We have rivers, which only rarely carry any water. We have brooks which from time to time become raging torrents. The waterway is whatever people choose to call it.
And yes, it's the same for pools, ponds, lakes, bays, seas, and oceans.
And it's the same for the boundaries between oceans. There is no particular point at which one ocean ends, and another begins.
Also for people. What's the difference between an "ordinary" and a "big" or "small" person? When does someone become "fat"? "Skinny"? Who is "older" or "old", and who is "young"? Who is "average" and who is "handsome" (or even "ugly")? There are no well-defined boundaries.
In some ways it's a bit frustrating to have such imprecision in language, particularly when in other contexts (land area measures, for instance) everything is so precise. Just an oddity of how our language has evolved.
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