[Koshkin]

More to the point, the idea that "you cannot separate a river from the ocean" seems to be ostensibly wrong - otherwise we wouldn't have the (very useful in practice) notions of 'river' and 'ocean' in the first place. If you look at a map, the differences stand out pretty clearly - e.g., a river has a starkly different topology from that of the ocean. So, no, these differences are not just figments of our imagination, as, in particular, everyday practice shows.

In general, the failure to perceive emergent phenomena as something different from the particular substrate, and consider it separately - for instance, the failure to see how nature is not just a bunch of atoms moving around, has a name - reductionism. It is a form of intellectual blindness (not to be confused with the ability to think abstractly).